Java null检查为什么使用==代替.equals()

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?

回答:

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

以上是 Java null检查为什么使用==代替.equals() 的全部内容, 来源链接: utcz.com/qa/413945.html

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