Void.java

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package java.lang;

/**
 * The {@code Void} class is an uninstantiable placeholder class to hold a reference to the {@code Class} object representing the Java keyword void.
 *
 * @author unascribed
 * @since 1.1
 *
 * Void类是一个不可实例化的占位符类，用于表示对Java关键字void的类对象的引用。
 *
 * 可以用于判断某个方法的返回值是否为void：
 * public void foo() {}
 * ...
 * if (getClass().getMethod("foo").getReturnType() == Void.TYPE)
 * ...
 *
 *
 * 也可用在泛型中，如需要一个返回值为void的泛型。
 * abstract class Foo<T> {
 *     abstract T bar();
 * }
 *
 * class Bar extends Foo<Void> {
 *     Void bar() {
 *         return (null);
 *     }
 * }
 *
 * 还可用在形参中，仅仅表示占位。
 */
public final class Void {
    
    /**
     * The {@code Class} object representing the pseudo-type corresponding to the keyword {@code void}.
     */
    @SuppressWarnings("unchecked")
    public static final Class<Void> TYPE = (Class<Void>) Class.getPrimitiveClass("void");
    
    /**
     * The Void class cannot be instantiated.
     */
    // 无法被实例化
    private Void() {
    }
}