描述：一元运算，接受一个T类型参数，输出一个与入参一模一样的值
源码：
package java.util.function;
/**
* Represents an operation on a single operand that produces a result of the
* same type as its operand. This is a specialization of {@code Function} for
* the case where the operand and result are of the same type.
*
* <p>This is a <a href="package-summary.html">functional interface</a>
* whose functional method is {@link #apply(Object)}.
*
* @param <T> the type of the operand and result of the operator
*
* @see Function
* @since 1.8
*/
@FunctionalInterface
public interface UnaryOperator<T> extends Function<T, T> {
/**
* Returns a unary operator that always returns its input argument.
*
* @param <T> the type of the input and output of the operator
* @return a unary operator that always returns its input argument
*/
static <T> UnaryOperator<T> identity() {
return t -> t;
}
}
测试代码：
System.out.println(UnaryOperator.identity().apply(10)); // 10
System.out.println(UnaryOperator.identity().apply(10.01)); // 10.01
System.out.println(UnaryOperator.identity().apply(false)); // false
System.out.println(UnaryOperator.identity().apply("10")); // 10
也可以预先定义好T类型的参数，测试代码如下
UnaryOperator<Integer> b = x->x.intValue(); // lambda表达式，这样就只能输入Integer类型了 System.out.println(b.apply(10));
