Java null String等于结果 [英] Java null String equals result

问题描述

请帮我看看java中的string.equals如何使用null值？异常有问题吗？
三种情况：

Please help me how does the string.equals in java work with null value? Is there some problem with exceptions? Three cases:

boolean result1,result2, result3;

    //1st case
    String string1 = null;
    String string2 = null;
    result = string1.equals(string2);
    //2nd case
    String string1 = "something";
    String string2 = null;
    result2 = string1.equals(string2);
    //3rd case 
    String string1 = null;
    String string2 = "something";
    result3 = string1.equals(string2);

结果的值是多少？我期待这个值：

What the values of results are? I expect this values:

result1为true;

result2为false;

result3是假的;

result1 is true;
result2 is false;
result3 is false;

推荐答案

你不能使用dereference（dot，'。'）运算符来如果该实例是 null ，则访问实例上的实例变量或调用方法。这样做会产生 NullPointerException 。

You cannot use the dereference (dot, '.') operator to access instance variables or call methods on an instance if that instance is null. Doing so will yield a NullPointerException.

通常的做法是使用你知道的非null值字符串比较。例如，something.equals（stringThatMayBeNull）。

It is common practice to use something you know to be non-null for string comparison. For example, "something".equals(stringThatMayBeNull).

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