Java null检查为什么使用==代替.equals（）

Question

在Java中，有人告诉我在执行null检查时应使用==而不是.equals（）。是什么原因呢？

Answer 1

他们是完全不同的两件事。==比较变量包含的对象引用（如果有）。根据相等性的含义.equals()检查两个对象是否相等。根据它们的契约，两个不同的对象实例完全有可能“相等”。还有一个小细节，因为这equals是一个方法，所以如果你尝试在引用上调用它null，则会得到一个NullPointerException。

例如：

class Foo {
    private int data;

    Foo(int d) {
        this.data = d;
    }

    @Override
    public boolean equals(Object other) {
        if (other == null || other.getClass() != this.getClass()) {
           return false;
        }
        return ((Foo)other).data == this.data;
    }

    /* In a real class, you'd override `hashCode` here as well */
}

Foo f1 = new Foo(5);
Foo f2 = new Foo(5);
System.out.println(f1 == f2);
// outputs false, they're distinct object instances

System.out.println(f1.equals(f2));
// outputs true, they're "equal" according to their definition

Foo f3 = null;
System.out.println(f3 == null);
// outputs true, `f3` doesn't have any object reference assigned to it

System.out.println(f3.equals(null));
// Throws a NullPointerException, you can't dereference `f3`, it doesn't refer to anything

System.out.println(f1.equals(f3));
// Outputs false, since `f1` is a valid instance but `f3` is null,
// so one of the first checks inside the `Foo#equals` method will
// disallow the equality because it sees that `other` == null