Java determines whether a character is empty

java determines whether a character is empty

The following summarizes four ways for you to determine whether a character is empty in java method, hope it helps you.

Method 1. The most popular method, intuitive and convenient, but very inefficient:

if(s == null || s.equals(""));

Method 2. Compare string lengths, efficient High, it is the best method I know:

if(s == null || s.length() == 0);

Method 3, a method only started to be provided by Java SE 6.0, is almost as efficient as Method 2, but for compatibility reasons, It is recommended to use method 2.

if(s == null || s.isEmpty());

Method 4. This is a relatively intuitive and simple method, and the efficiency is also very high. It is almost as efficient as methods 2 and 3:

if (s == null || s == "");

Note: s == null is necessary. (Recommended tutorial: java tutorial)

If the String type is null, operations such as equals(String) or length() will throw a java.lang.NullPointerException.

And the order of s==null must appear in front, otherwise it will also throw java.lang.NullPointerException.

The following Java code:

String str = null;
f(str.equals("") || str == null){//会抛出异常
    System.out.println("success");
}

Supplement:

1. Empty string Differences from null

1. Type

null represents the value of an object, not a string. For example, when declaring a reference to an object, String a = null;

"" represents an empty string, which means its length is 0. For example, declaring a string String str = "" ;

2. Memory allocation

String str = null ; means declaring a reference to a string object, but the pointer is null, that is to say, it is still It does not point to any memory space;

String str = ""; means declaring a reference of string type, whose value is ""empty string. This str reference points to the memory space of the empty string;

In Java, variables and reference variables are stored in the stack (stack), while objects (generated by new) are stored in the heap (heap):

It is as follows:

String str = new String("abc") ;

ps: =The one on the left is stored in the stack (stack), =The one on the right is stored in the heap (heap).

3. Sample program:

public class String_Demo01 {
    public static void main(String[] args) {        
        String str1 = new String() ;
        String str2 = null ;
        String str3 = "" ;
        System.out.println(str1==str2);                //内存地址的比较，返回false
        System.out.println(str1.equals(str2));         //值的比较，返回false
        System.out.println(str2==str3);                //内存地址的比较，返回false
        System.out.println(str3.equals(str2));         //值的比较，返回false
        System.out.println(str1==str3);                //内存地址的比较，返回false
        System.out.println(str1.equals(str3));         //值的比较，返回true
    }
}

Through the above program, the following conclusions can be drawn:

The values ​​of string objects and null are not equal, and the memory addresses are not equal either. ;

The value of the empty string object is not equal to null, and the memory address is not equal;

new String() creates a string object whose default value is "" (String type member The initial value of the variable is null)

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