Java 布尔值和 NULL 的处理

Question

JAVA 中的布尔对象可以有 3 个值 True、False、NULL

public class First {

    public static void main(String args[])
    {
        System.out.println("equals(new Boolean(\"True\"),True) :: " + isEqual(new Boolean("True"), true));
        System.out.println("equals(new Boolean(\"False\"), new Boolean(null)) :: " + isEqual(new Boolean("False"), new Boolean(null)));
        System.out.println("equals(new Boolean(\"False\"), null)) :: " + isEqual(new Boolean("False"), null));
    }

    static boolean isEqual(Boolean a, Boolean b)
    {
        return a.equals(b);
    }
}

以上代码的输出是

equals(new Boolean("True"),True) :: true
equals(new Boolean("False"), new Boolean(null)) :: true
equals(new Boolean("False"), null)) :: false

请解释为什么案例 2 returns 正确但案例 3 returns 错误

Answer 1

如果你查看 Booleanclass 的源代码，你可以看到传递一个 null 值 returns false:

private static boolean toBoolean(String name) { 
    return ((name != null) && name.equalsIgnoreCase("true"));
 }

Answer 2

这是因为 Boolean 的构造函数，如果提供 null 将分配一个表示值 false

的布尔对象

阅读此处：http://docs.oracle.com/javase/7/docs/api/java/lang/Boolean.html

public Boolean(String s)

Allocates a Boolean object representing the value true if the string argument is not null and is equal, ignoring case, to the string "true". Otherwise, allocate a Boolean object representing the value false. Examples: new Boolean("True") produces a Boolean object that represents true. new Boolean("yes") produces a Boolean object that represents false. Parameters:s - the string to be converted to a Boolean.

Java 布尔值和 NULL 的处理

Java handling of Boolean and NULL

java

boolean