Java null 检查为什么使用 == 而不是 .equals()
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Java null check why use == instead of .equals()
提问by Jim Jeffries
In Java I am told that when doing a null check one should use == instead of .equals(). What are the reasons for this?
在 Java 中，我被告知在进行空检查时应该使用 == 而不是 .equals()。造成这种情况的原因是什么？
采纳答案by T.J. Crowder
They're two completely different things. ==compares the object reference, if any, contained by a variable. .equals()checks to see if two objects are equalaccording to their contract for what equality means. It's entirely possible for two distinct object instances to be "equal" according to their contract. And then there's the minor detail that since equalsis a method, if you try to invoke it on a nullreference, you'll get a NullPointerException.
它们是完全不同的两种东西。==比较变量包含的对象引用（如果有）。.equals()检查两个对象是否根据它们的合同相等，以了解相等的含义。根据合约，两个不同的对象实例完全有可能“相等”。然后equals是一个小细节，因为它是一个方法，如果你尝试在引用上调用它null，你会得到一个NullPointerException.
For instance:
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
回答by Jigar Joshi
if you invoke .equals()on nullyou will get NullPointerException
如果你调用.equals()上null，你会得到NullPointerException
So it is always advisble to check nullity before invoking method where ever it applies
因此，始终建议在调用任何适用的方法之前检查无效性
if(str!=null && str.equals("hi")){
//str contains hi
}
Also See
另见
回答by Jé Queue
If an Object variable is null, one cannot call an equals() method upon it, thus an object reference check of null is proper.
如果 Object 变量为 null，则不能对其调用 equals() 方法，因此对 null 的对象引用检查是正确的。
回答by Nick Orton
foo.equals(null)
What happens if foo is null?
如果 foo 为空会发生什么？
You get a NullPointerException.
你得到一个 NullPointerException。
回答by A_M
If you try calling equals on a null object reference, then you'll get a null pointer exception thrown.
如果您尝试在空对象引用上调用 equals，则会抛出空指针异常。
回答by danny.lesnik
Because equal is a function derived from Object class, this function compares items of the class. if you use it with null it will return false cause cause class content is not null. In addition == compares reference to an object.
因为equal是从Object类派生的函数，所以这个函数比较类的项。如果您将它与 null 一起使用，它将返回 false 原因导致类内容不为 null。另外 == 比较对对象的引用。
回答by DixonD
回答by user523859
In Java 0 or null are simple types and not objects.
在 Java 中 0 或 null 是简单类型而不是对象。
The method equals() is not built for simple types. Simple types can be matched with ==.
方法 equals() 不是为简单类型构建的。简单类型可以与 == 匹配。
回答by Kevin Orriss
You could always do
你总能做到
if (str == null || str.equals(null))
This will first check the object reference and then check the object itself providing the reference isnt null.
这将首先检查对象引用，然后检查对象本身提供的引用不为空。
回答by AKT
If we use=> .equals method
如果我们使用=> .equals 方法
if(obj.equals(null))
// Which mean null.equals(null) when obj will be null.
When your obj will be null it will throw Null Point Exception.
当您的 obj 为 null 时，它将抛出 Null Point Exception。
so we should use ==
所以我们应该使用 ==
if(obj == null)
it will compare the references.
它将比较参考。
