net.bytebuddy.implementation

Class InvokeDynamic

java.lang.Object

net.bytebuddy.implementation.InvokeDynamic

All Implemented Interfaces:

Implementation

Direct Known Subclasses:

InvokeDynamic.AbstractDelegator

public class InvokeDynamic
extends Object
implements Implementation

An implementation that applies a dynamic method invocation.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
protected static class	InvokeDynamic.AbstractDelegator An abstract delegator that allows to specify a configuration for any specification of an argument.
protected class	InvokeDynamic.Appender The byte code appender to be used by the InvokeDynamic implementation.
protected static interface	InvokeDynamic.InvocationProvider An invocation provider is responsible for loading the arguments of the invoked method onto the operand stack and for creating the actual invoke dynamic instruction.
protected static interface	InvokeDynamic.TerminationHandler A termination handler is responsible to handle the return value of a method that is invoked via a InvokeDynamic.
static class	InvokeDynamic.WithImplicitArguments Representation of an InvokeDynamic implementation where the bootstrapped method is passed a this reference, if available, and any arguments of the instrumented method.
static class	InvokeDynamic.WithImplicitArgumentType An invoke dynamic implementation where the last argument is an implicitly typed method argument.
static class	InvokeDynamic.WithImplicitFieldType A step in the invoke dynamic domain specific language that allows to explicitly specify a field type for a reference value.
static class	InvokeDynamic.WithImplicitTarget Representation of an InvokeDynamic implementation where the bootstrapped method is passed a this reference, if available, and any arguments of the instrumented method and where the invocation target is implicit.

Nested classes/interfaces inherited from interface net.bytebuddy.implementation.Implementation

Implementation.Compound, Implementation.Context, Implementation.Simple, Implementation.SpecialMethodInvocation, Implementation.Target

Field Summary

Fields

Modifier and Type	Field and Description
protected Assigner	assigner The assigner to be used.
protected MethodDescription.InDefinedShape	bootstrapMethod The bootstrap method.
protected List<?>	handleArguments The arguments that are provided to the bootstrap method.
protected InvokeDynamic.InvocationProvider	invocationProvider The target provided that identifies the method to be bootstrapped.
protected InvokeDynamic.TerminationHandler	terminationHandler A handler that handles the method return.
protected Assigner.Typing	typing Indicates if dynamic type castings should be attempted for incompatible assignments.

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	InvokeDynamic(MethodDescription.InDefinedShape bootstrapMethod, List<?> handleArguments, InvokeDynamic.InvocationProvider invocationProvider, InvokeDynamic.TerminationHandler terminationHandler, Assigner assigner, Assigner.Typing typing) Creates a new invoke dynamic implementation.

Method Summary

Modifier and Type	Method and Description
Implementation	andThen(Implementation implementation) Applies this invoke dynamic implementation and removes the return value of the bootstrapped method from the operand stack before applying the provided implementation.
ByteCodeAppender	appender(Implementation.Target implementationTarget) Creates a byte code appender that determines the implementation of the instrumented type's methods.
static InvokeDynamic.WithImplicitTarget	bootstrap(Constructor<?> constructor, List<?> rawArguments) Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap constructor.
static InvokeDynamic.WithImplicitTarget	bootstrap(Constructor<?> constructor, Object... rawArgument) Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap constructor.
static InvokeDynamic.WithImplicitTarget	bootstrap(MethodDescription.InDefinedShape bootstrapMethod, List<?> rawArguments) Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap method or constructor.
static InvokeDynamic.WithImplicitTarget	bootstrap(MethodDescription.InDefinedShape bootstrapMethod, Object... rawArgument) Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap method or constructor.
static InvokeDynamic.WithImplicitTarget	bootstrap(Method method, List<?> rawArguments) Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap method.
static InvokeDynamic.WithImplicitTarget	bootstrap(Method method, Object... rawArgument) Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap method.
boolean	equals(Object other)
protected InvokeDynamic.InvocationProvider	getInvocationProvider() Returns the invocation provider to be used for equals and hash code calculations.
int	hashCode()
InstrumentedType	prepare(InstrumentedType instrumentedType) During the preparation phase of an implementation, implementations are eligible to adding fields or methods to the currently instrumented type.
String	toString()
InvokeDynamic	withArgument(int... index) Passes parameters of the instrumented method to the bootstrapped method.
InvokeDynamic.WithImplicitArgumentType	withArgument(int index) Passes a parameter of the instrumented method to the bootstrapped method.
InvokeDynamic	withAssigner(Assigner assigner, Assigner.Typing typing) Instructs this implementation to use the provided assigner and decides if the assigner should apply dynamic typing.
InvokeDynamic	withBooleanValue(boolean... value) Requires the bootstrap method to bootstrap a method that takes the specified boolean arguments as its next parameters.
InvokeDynamic	withByteValue(byte... value) Requires the bootstrap method to bootstrap a method that takes the specified byte arguments as its next parameters.
InvokeDynamic	withCharacterValue(char... value) Requires the bootstrap method to bootstrap a method that takes the specified char arguments as its next parameters.
InvokeDynamic	withDoubleValue(double... value) Requires the bootstrap method to bootstrap a method that takes the specified double arguments as its next parameters.
InvokeDynamic	withEnumeration(EnumerationDescription... enumerationDescription) Hands the provided enumerations to the dynamically bound method.
InvokeDynamic	withField(String... fieldName) Passes the values of the specified fields to the bootstrap method.
InvokeDynamic	withFloatValue(float... value) Requires the bootstrap method to bootstrap a method that takes the specified float arguments as its next parameters.
InvokeDynamic	withImplicitAndMethodArguments() Adds a potential this reference and all method arguments to the the bootstrapped method.
InvokeDynamic	withInstance(JavaInstance... javaInstance) Hands the provided Java instance to the dynamically bound method.
InvokeDynamic	withInstanceField(String fieldName, Class<?> fieldType) Passes the value of the specified instance field to the bootstrapped method.
InvokeDynamic	withInstanceField(String fieldName, TypeDescription fieldType) Passes the value of the specified instance field to the bootstrapped method.
InvokeDynamic	withIntegerValue(int... value) Requires the bootstrap method to bootstrap a method that takes the specified int arguments as its next parameters.
InvokeDynamic	withLongValue(long... value) Requires the bootstrap method to bootstrap a method that takes the specified long arguments as its next parameters.
InvokeDynamic	withMethodArguments() Adds all method arguments to the the bootstrapped method.
InvokeDynamic	withNullValue(Class<?>... type) Passes null values of the given types to the bootstrapped method.
InvokeDynamic	withNullValue(TypeDescription... typeDescription) Passes null values of the given types to the bootstrapped method.
InvokeDynamic	withReference(Object... value) Requires the bootstrap method to bootstrap a method that takes the specified arguments as its next parameters.
InvokeDynamic.WithImplicitFieldType	withReference(Object value) Requires the bootstrap method to bootstrap a method that takes the specified argument as its next parameter while allowing to specify the value to be of a different type than the actual instance type.
InvokeDynamic	withShortValue(short... value) Requires the bootstrap method to bootstrap a method that takes the specified short arguments as its next parameters.
InvokeDynamic	withThis(Class<?>... type) Passes references to this onto the operand stack where the instance is represented as the given types.
InvokeDynamic	withThis(TypeDescription... typeDescription) Passes references to this onto the operand stack where the instance is represented as the given types.
InvokeDynamic	withType(TypeDescription... typeDescription) Hands the provided types to the dynamically bound method.
InvokeDynamic	withValue(Object... value) Requires the bootstrap method to bootstrap a method that takes the specified arguments as its next parameters.

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Field Detail

bootstrapMethod

protected final MethodDescription.InDefinedShape bootstrapMethod

The bootstrap method.

handleArguments

protected final List<?> handleArguments

The arguments that are provided to the bootstrap method.

invocationProvider

protected final InvokeDynamic.InvocationProvider invocationProvider

The target provided that identifies the method to be bootstrapped.

terminationHandler

protected final InvokeDynamic.TerminationHandler terminationHandler

A handler that handles the method return.

assigner

protected final Assigner assigner

The assigner to be used.

typing

protected final Assigner.Typing typing

Indicates if dynamic type castings should be attempted for incompatible assignments.

Constructor Detail

InvokeDynamic

protected InvokeDynamic(MethodDescription.InDefinedShape bootstrapMethod,
                        List<?> handleArguments,
                        InvokeDynamic.InvocationProvider invocationProvider,
                        InvokeDynamic.TerminationHandler terminationHandler,
                        Assigner assigner,
                        Assigner.Typing typing)

Creates a new invoke dynamic implementation.

Parameters:

bootstrapMethod - The bootstrap method.

handleArguments - The arguments that are provided to the bootstrap method.

invocationProvider - The target provided that identifies the method to be bootstrapped.

terminationHandler - A handler that handles the method return.

assigner - The assigner to be used.

typing - Indicates if dynamic type castings should be attempted for incompatible assignments.

Method Detail

bootstrap

public static InvokeDynamic.WithImplicitTarget bootstrap(Method method,
                                                         Object... rawArgument)

Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap method.

Parameters:

method - The bootstrap method that is used to link the instrumented method.

rawArgument - The arguments that are handed to the bootstrap method. Any argument must be saved in the constant pool, i.e. primitive types (represented as their wrapper types) with a size of at least 32 bit, String types, Class types as well as MethodType and MethodHandle instances. In order to avoid class loading, it is also possible to supply unloaded types as TypeDescription, JavaInstance.MethodHandle or JavaInstance.MethodType instances. instrumented method are passed to the bootstrapped method unless explicit parameters are specified.

Returns:

An implementation where a this reference, if available, and all arguments of the instrumented method are passed to the bootstrapped method unless explicit parameters are specified.

bootstrap

public static InvokeDynamic.WithImplicitTarget bootstrap(Method method,
                                                         List<?> rawArguments)

Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap method.

Parameters:

method - The bootstrap method that is used to link the instrumented method.

rawArguments - The arguments that are handed to the bootstrap method. Any argument must be saved in the constant pool, i.e. primitive types (represented as their wrapper types) with a size of at least 32 bit, String types, Class types as well as MethodType and MethodHandle instances. In order to avoid class loading, it is also possible to supply unloaded types as TypeDescription, JavaInstance.MethodHandle or JavaInstance.MethodType instances. instrumented method are passed to the bootstrapped method unless explicit parameters are specified.

Returns:

An implementation where a this reference, if available, and all arguments of the instrumented method are passed to the bootstrapped method unless explicit parameters are specified.

bootstrap

public static InvokeDynamic.WithImplicitTarget bootstrap(Constructor<?> constructor,
                                                         Object... rawArgument)

Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap constructor.

Parameters:

constructor - The bootstrap constructor that is used to link the instrumented method.

rawArgument - The arguments that are handed to the bootstrap method. Any argument must be saved in the constant pool, i.e. primitive types (represented as their wrapper types) with a size of at least 32 bit, String types, Class types as well as MethodType and MethodHandle instances. In order to avoid class loading, it is also possible to supply unloaded types as TypeDescription, JavaInstance.MethodHandle or JavaInstance.MethodType instances.

Returns:

An implementation where a this reference, if available, and all arguments of the instrumented method are passed to the bootstrapped method unless explicit parameters are specified.

bootstrap

public static InvokeDynamic.WithImplicitTarget bootstrap(Constructor<?> constructor,
                                                         List<?> rawArguments)

Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap constructor.

Parameters:

constructor - The bootstrap constructor that is used to link the instrumented method.

rawArguments - The arguments that are handed to the bootstrap method. Any argument must be saved in the constant pool, i.e. primitive types (represented as their wrapper types) with a size of at least 32 bit, String types, Class types as well as MethodType and MethodHandle instances. In order to avoid class loading, it is also possible to supply unloaded types as TypeDescription, JavaInstance.MethodHandle or JavaInstance.MethodType instances.

Returns:

An implementation where a this reference, if available, and all arguments of the instrumented method are passed to the bootstrapped method unless explicit parameters are specified.

bootstrap

public static InvokeDynamic.WithImplicitTarget bootstrap(MethodDescription.InDefinedShape bootstrapMethod,
                                                         Object... rawArgument)

Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap method or constructor.

Parameters:

bootstrapMethod - The bootstrap method or constructor that is used to link the instrumented method.

rawArgument - The arguments that are handed to the bootstrap method. Any argument must be saved in the constant pool, i.e. primitive types (represented as their wrapper types) with a size of at least 32 bit, String types, Class types as well as MethodType and MethodHandle instances. In order to avoid class loading, it is also possible to supply unloaded types as TypeDescription, JavaInstance.MethodHandle or JavaInstance.MethodType instances.

Returns:

An implementation where a this reference, if available, and all arguments of the instrumented method are passed to the bootstrapped method unless explicit parameters are specified.

bootstrap

public static InvokeDynamic.WithImplicitTarget bootstrap(MethodDescription.InDefinedShape bootstrapMethod,
                                                         List<?> rawArguments)

Implements the instrumented method with a dynamic method invocation which is linked at runtime using the specified bootstrap method or constructor.

Parameters:

bootstrapMethod - The bootstrap method or constructor that is used to link the instrumented method.

rawArguments - The arguments that are handed to the bootstrap method. Any argument must be saved in the constant pool, i.e. primitive types (represented as their wrapper types) with a size of at least 32 bit, String types, Class types as well as MethodType and MethodHandle instances. In order to avoid class loading, it is also possible to supply unloaded types as TypeDescription, JavaInstance.MethodHandle or JavaInstance.MethodType instances.

Returns:

An implementation where a this reference, if available, and all arguments of the instrumented method are passed to the bootstrapped method unless explicit parameters are specified.

withBooleanValue

public InvokeDynamic withBooleanValue(boolean... value)

Requires the bootstrap method to bootstrap a method that takes the specified boolean arguments as its next parameters.

Parameters:

value - The arguments to pass to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withByteValue

public InvokeDynamic withByteValue(byte... value)

Requires the bootstrap method to bootstrap a method that takes the specified byte arguments as its next parameters.

Parameters:

value - The arguments to pass to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withShortValue

public InvokeDynamic withShortValue(short... value)

Requires the bootstrap method to bootstrap a method that takes the specified short arguments as its next parameters.

Parameters:

value - The arguments to pass to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withCharacterValue

public InvokeDynamic withCharacterValue(char... value)

Requires the bootstrap method to bootstrap a method that takes the specified char arguments as its next parameters.

Parameters:

value - The arguments to pass to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withIntegerValue

public InvokeDynamic withIntegerValue(int... value)

Requires the bootstrap method to bootstrap a method that takes the specified int arguments as its next parameters.

Parameters:

value - The arguments to pass to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withLongValue

public InvokeDynamic withLongValue(long... value)

Requires the bootstrap method to bootstrap a method that takes the specified long arguments as its next parameters.

Parameters:

value - The arguments to pass to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withFloatValue

public InvokeDynamic withFloatValue(float... value)

Requires the bootstrap method to bootstrap a method that takes the specified float arguments as its next parameters.

Parameters:

value - The arguments to pass to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withDoubleValue

public InvokeDynamic withDoubleValue(double... value)

Requires the bootstrap method to bootstrap a method that takes the specified double arguments as its next parameters.

Parameters:

value - The arguments to pass to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withValue

public InvokeDynamic withValue(Object... value)

Requires the bootstrap method to bootstrap a method that takes the specified arguments as its next parameters. Note that any primitive parameters are passed as their wrapper types. Furthermore, values that can be stored in the instrumented class's constant pool might be of different object identity when passed to the bootstrapped method or might not be visible to the the created class what later results in a runtime error.

Parameters:

value - The arguments to pass to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withReference

public InvokeDynamic.WithImplicitFieldType withReference(Object value)

Requires the bootstrap method to bootstrap a method that takes the specified argument as its next parameter while allowing to specify the value to be of a different type than the actual instance type.

Parameters:

value - The arguments to pass to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withReference

public InvokeDynamic withReference(Object... value)

Requires the bootstrap method to bootstrap a method that takes the specified arguments as its next parameters. Note that any primitive parameters are passed as their wrapper types. Any value that is passed to the bootstrapped method is guaranteed to be of the same object identity.

Parameters:

value - The arguments to pass to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withType

public InvokeDynamic withType(TypeDescription... typeDescription)

Hands the provided types to the dynamically bound method. The type is stored in the generated class's constant pool and is loaded at invocation time. For this to be possible, the created class's class loader must be able to see the provided type.

Parameters:

typeDescription - The classes to provide to the bound method as an argument.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified type.

withEnumeration

public InvokeDynamic withEnumeration(EnumerationDescription... enumerationDescription)

Hands the provided enumerations to the dynamically bound method. The enumeration values are read from the enumeration class on demand. For this to be possible, the created class's class loader must be able to see the enumeration type.

Parameters:

enumerationDescription - The enumeration values to provide to the bound method as an argument.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified enumerations.

withInstance

public InvokeDynamic withInstance(JavaInstance... javaInstance)

Hands the provided Java instance to the dynamically bound method. The instance is stored in the generated class's constant pool and is loaded at invocation time. For this to be possible, the created class's class loader must be able to create the provided Java instance.

Parameters:

javaInstance - The Java instance to provide to the bound method as an argument.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified Java instance.

withNullValue

public InvokeDynamic withNullValue(Class<?>... type)

Passes null values of the given types to the bootstrapped method.

Parameters:

type - The type that the null values should represent.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withNullValue

public InvokeDynamic withNullValue(TypeDescription... typeDescription)

Passes null values of the given types to the bootstrapped method.

Parameters:

typeDescription - The type that the null values should represent.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withArgument

public InvokeDynamic withArgument(int... index)

Passes parameters of the instrumented method to the bootstrapped method.

Parameters:

index - The indices of the parameters that should be passed to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withArgument

public InvokeDynamic.WithImplicitArgumentType withArgument(int index)

Passes a parameter of the instrumented method to the bootstrapped method.

Parameters:

index - The index of the parameter that should be passed to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified argument with its implicit type.

withThis

public InvokeDynamic withThis(Class<?>... type)

Passes references to this onto the operand stack where the instance is represented as the given types.

Parameters:

type - The types as which the this reference of the intercepted method should be masked.

Returns:

This implementation where this references are passed as the next arguments.

withThis

public InvokeDynamic withThis(TypeDescription... typeDescription)

Passes references to this onto the operand stack where the instance is represented as the given types.

Parameters:

typeDescription - The types as which the this reference of the intercepted method should be masked.

Returns:

This implementation where this references are passed as the next arguments.

withMethodArguments

public InvokeDynamic withMethodArguments()

Adds all method arguments to the the bootstrapped method.

Returns:

This invoke dynamic implementation with all parameters of the instrumented method added.

withImplicitAndMethodArguments

public InvokeDynamic withImplicitAndMethodArguments()

Adds a potential this reference and all method arguments to the the bootstrapped method.

Returns:

This invoke dynamic implementation with a potential this reference and all parameters of the instrumented method added.

withInstanceField

public InvokeDynamic withInstanceField(String fieldName,
                                       Class<?> fieldType)

Passes the value of the specified instance field to the bootstrapped method. The field value of the created private field must be set manually on any instrumented instance.

Parameters:

fieldName - The name of the field.

fieldType - The type of the field.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the value of the defined field.

withInstanceField

public InvokeDynamic withInstanceField(String fieldName,
                                       TypeDescription fieldType)

Passes the value of the specified instance field to the bootstrapped method. The field value of the created private field must be set manually on any instrumented instance.

Parameters:

fieldName - The name of the field.

fieldType - The type of the field.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the value of the defined field.

withField

public InvokeDynamic withField(String... fieldName)

Passes the values of the specified fields to the bootstrap method. Any of the specified fields must already exist for the instrumented type.

Parameters:

fieldName - The names of the fields to be passed to the bootstrapped method.

Returns:

This invoke dynamic implementation where the bootstrapped method is passed the specified arguments.

withAssigner

public InvokeDynamic withAssigner(Assigner assigner,
                                  Assigner.Typing typing)

Instructs this implementation to use the provided assigner and decides if the assigner should apply dynamic typing.

Parameters:

assigner - The assigner to use.

typing - true if the assigner should attempt dynamic typing.

Returns:

The invoke dynamic instruction where the given assigner and dynamic-typing directive are applied.

andThen

public Implementation andThen(Implementation implementation)

Applies this invoke dynamic implementation and removes the return value of the bootstrapped method from the operand stack before applying the provided implementation.

Parameters:

implementation - The implementation to apply after executing the dynamic method invocation.

Returns:

An implementation that first applies this implementation and then the provided one.

prepare

public InstrumentedType prepare(InstrumentedType instrumentedType)

Description copied from interface: Implementation

During the preparation phase of an implementation, implementations are eligible to adding fields or methods to the currently instrumented type. All methods that are added by this implementation are required to be implemented by the ByteCodeAppender that is emitted on the call to Implementation.appender(Implementation.Target) call. On this method call, loaded type initializers can also be added to the instrumented type.

Specified by:

prepare in interface Implementation

Parameters:

instrumentedType - The instrumented type that is the basis of the ongoing instrumentation.

Returns:

The instrumented type with any applied changes, if any.

appender

public ByteCodeAppender appender(Implementation.Target implementationTarget)

Description copied from interface: Implementation

Creates a byte code appender that determines the implementation of the instrumented type's methods.

Specified by:

appender in interface Implementation

Parameters:

implementationTarget - The target of the current implementation.

Returns:

A byte code appender for implementing methods delegated to this implementation. This byte code appender is also responsible for handling methods that were added by this implementation on the call to Implementation.prepare(InstrumentedType).

getInvocationProvider

protected InvokeDynamic.InvocationProvider getInvocationProvider()

Returns the invocation provider to be used for equals and hash code calculations.

Returns:

The invocation provider that represents this instance.

equals

public boolean equals(Object other)

Overrides:

equals in class Object

hashCode

public int hashCode()

Overrides:

hashCode in class Object

toString

public String toString()

Overrides:

toString in class Object