equals方法变量和常量位置区别

时间:2021-08-13 21:13:03

对于字符串比较,我的习惯用法是   变量.equals(常量)

  比如:     a.equals("a")

  今天看视频才知道变量在前面与后面有很大影响,正确的写法是常量放前面(可以避免变量为null引起的异常),变量放后面,则上面需要改为("a").equals(a)。

  对于JDK API上字符串的equals的官方解释是:

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" alt="" />

比如:

public class equalsTest {
@SuppressWarnings("rawtypes")
public static void main(String args[]) {
List list = new ArrayList();
String str = null;
System.out.println("0".equals(str));
System.out.println(str.equals("0"));
System.out.println("0".equals(list.get(2)));
System.out.println(list.get(2).equals("0"));
}
}

上面依次输出:

   false

   java.lang.NullPointerException

   java.lang.IndexOutOfBoundsException: Index: 2, Size: 0

   java.lang.IndexOutOfBoundsException: Index: 2, Size: 0

  对于判断是否为空  str!=null,这个应该变量放前面后面一般没影响,养成好的习惯的话变量也放在后面。一般对一个字符串进行判断首先进行非空判断,然后进行字符串内容判断,一个正确的例子:

  null !=string && !("").equals(string.trim()) && !("null").equals(string);