Java 空检查为什么使用 == 而不是 .equals（）

它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）。 检查两个对象是否根据其合约是否相等，以了解相等的含义。两个不同的对象实例完全有可能根据其协定“相等”。然后是一个小细节，因为这是一个方法，如果你尝试在引用上调用它，你会得到一个.==.equals()equalsnullNullPointerException

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

如果你调用你会得到.equals()nullNullPointerException

因此，始终建议在调用方法之前检查空性

if(str!=null && str.equals("hi")){
 //str contains hi
}  

另请参见

• Java 中的 diff-between-equals-and ==