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？
最佳答案
他们是两个完全不同的东西。 ==比较变量包含的对象引用（如果有）。 .equals()根据两个对象的约定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equals是一种方法，所以如果您尝试在null引用上调用它，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