# Class Guards

java.lang.Object
jdk.dynalink.linker.support.Guards

public final class Guards extends Object
Utility methods for creating typical guards for `MethodHandles.guardWithTest(MethodHandle, MethodHandle, MethodHandle)` and for adjusting their method types.

Since:
  9

## Method Summary

Modifier and Type
Method
Description
`static MethodHandle`
`asType(MethodHandle test, MethodType type)`
Takes a method handle intended to be used as a guard, and adapts it to the requested type, but returning a boolean.
`static MethodHandle`
`asType(LinkerServices linkerServices, MethodHandle test, MethodType type)`
Takes a method handle intended to be used as a guard, and adapts it to the requested type, but returning a boolean.
`static MethodHandle`
`getClassGuard(Class<?> clazz)`
Creates a guard method that tests its only argument for being of an exact particular class.
`static MethodHandle`
`getIdentityGuard(Object obj)`
Creates a guard method that tests its only argument for being referentially identical to another object
`static MethodHandle`
`getInstanceOfGuard(Class<?> clazz)`
Creates a guard method that tests its only argument for being an instance of a particular class.
`static MethodHandle`
`isArray(int pos, MethodType type)`
Creates a method handle that returns true if the argument in the specified position is a Java array.
`static MethodHandle`
`isInstance(Class<?> clazz, int pos, MethodType type)`
Creates a method handle with arguments of a specified type, but with boolean return value.
`static MethodHandle`
`isInstance(Class<?> clazz, MethodType type)`
Creates a method handle with arguments of a specified type, but with boolean return value.
`static MethodHandle`
`isNotNull()`
Returns a guard that tests whether the first argument is not null.
`static MethodHandle`
`isNull()`
Returns a guard that tests whether the first argument is null.
`static MethodHandle`
`isOfClass(Class<?> clazz, MethodType type)`
Creates a guard method handle with arguments of a specified type, but with boolean return value.

### Methods declared in class Object

`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

## Method Details

### isOfClass

public static MethodHandle isOfClass(Class<?> clazz, MethodType type)
Creates a guard method handle with arguments of a specified type, but with boolean return value. When invoked, it returns true if the first argument is of the specified class (exactly of it, not a subclass). The rest of the arguments will be ignored.

Parameters:
  `clazz` - the class of the first argument to test for
  `type` - the method type
Returns:
  a method handle testing whether its first argument is of the specified class.

### isInstance

public static MethodHandle isInstance(Class<?> clazz, MethodType type)
Creates a method handle with arguments of a specified type, but with boolean return value. When invoked, it returns true if the first argument is instance of the specified class or its subclass). The rest of the arguments will be ignored.

Parameters:
  `clazz` - the class of the first argument to test for
  `type` - the method type
Returns:
  a method handle testing whether its first argument is of the specified class or subclass.

### isInstance

public static MethodHandle isInstance(Class<?> clazz, int pos, MethodType type)
Creates a method handle with arguments of a specified type, but with boolean return value. When invoked, it returns true if the n'th argument is instance of the specified class or its subclass). The rest of the arguments will be ignored.

Parameters:
  `clazz` - the class of the first argument to test for
  `pos` - the position on the argument list to test
  `type` - the method type
Returns:
  a method handle testing whether its first argument is of the specified class or subclass.

### isArray

public static MethodHandle isArray(int pos, MethodType type)
Creates a method handle that returns true if the argument in the specified position is a Java array.

Parameters:
  `pos` - the position in the argument lit
  `type` - the method type of the handle
Returns:
  a method handle that returns true if the argument in the specified position is a Java array; the rest of the arguments are ignored.

### asType

public static MethodHandle asType(MethodHandle test, MethodType type)
Takes a method handle intended to be used as a guard, and adapts it to the requested type, but returning a boolean. Applies `MethodHandle.asType(MethodType)` to convert types and uses `MethodHandles.dropArguments(MethodHandle, int, Class...)` to match the requested type arity.

Parameters:
  `test` - the test method handle
  `type` - the type to adapt the method handle to
Returns:
  the adapted method handle

### asType

public static MethodHandle asType(LinkerServices linkerServices, MethodHandle test, MethodType type)
Takes a method handle intended to be used as a guard, and adapts it to the requested type, but returning a boolean. Applies `LinkerServices.asType(MethodHandle, MethodType)` to convert types and uses `MethodHandles.dropArguments(MethodHandle, int, Class...)` to match the requested type arity.

Parameters:
  `linkerServices` - the linker services to use for type conversions
  `test` - the test method handle
  `type` - the type to adapt the method handle to
Returns:
  the adapted method handle

### getClassGuard

public static MethodHandle getClassGuard(Class<?> clazz)
Creates a guard method that tests its only argument for being of an exact particular class.

Parameters:
  `clazz` - the class to test for.
Returns:
  the desired guard method.

### getInstanceOfGuard

public static MethodHandle getInstanceOfGuard(Class<?> clazz)
Creates a guard method that tests its only argument for being an instance of a particular class.

Parameters:
  `clazz` - the class to test for.
Returns:
  the desired guard method.

### getIdentityGuard

public static MethodHandle getIdentityGuard(Object obj)
Creates a guard method that tests its only argument for being referentially identical to another object

Parameters:
  `obj` - the object used as referential identity test
Returns:
  the desired guard method.

### isNull

public static MethodHandle isNull()
Returns a guard that tests whether the first argument is null.

Returns:
  a guard that tests whether the first argument is null.

### isNotNull

public static MethodHandle isNotNull()
Returns a guard that tests whether the first argument is not null.

Returns:
  a guard that tests whether the first argument is not null.