public class Animal {
public static void foo() {
System.out.println("Animal");
}
}
public class Cat extends Animal {
public static void foo() { // hides Animal.foo()
System.out.println("Cat");
}
}
在这里，Cat.foo()据说藏起来了Animal.foo()。隐藏不像覆盖那样工作，因为静态方法不是多态的。因此，将发生以下情况：
Animal.foo(); // prints Animal
Cat.foo(); // prints Cat
Animal a = new Animal();
Animal b = new Cat();
Cat c = new Cat();
Animal d = null;
a.foo(); // should not be done. Prints Animal because the declared type of a is Animal
b.foo(); // should not be done. Prints Animal because the declared type of b is Animal
c.foo(); // should not be done. Prints Cat because the declared type of c is Cat
d.foo(); // should not be done. Prints Animal because the declared type of d is Animal
在实例而不是类上调用静态方法是一种非常糟糕的做法，绝不应该这样做。
将其与实例方法进行比较，实例方法是多态的，因此被覆盖。调用的方法取决于对象的具体运行时类型：
public class Animal {
public void foo() {
System.out.println("Animal");
}
}
public class Cat extends Animal {
public void foo() { // overrides Animal.foo()
System.out.println("Cat");
}
}
然后将发生以下情况：
Animal a = new Animal();
Animal b = new Cat();
Animal c = new Cat();
Animal d = null;
a.foo(); // prints Animal
b.foo(); // prints Cat
c.foo(); // prints Cat
d.foo(): // throws NullPointerException
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