Java 判断字符串是否为空[通俗易懂]

String 对象是否为空值判断几种方法

import java.io.File;

public class StringTest { 
   

    public static void main(String[] args) {
        String str = "1111111222222222a";

        if(str == null || "".equals(str))
            System.out.print("str为空1\n");
        if(str == null || str.length() <= 0)
            System.out.print("str为空2\n");
        if(str == null || str.isEmpty())
            System.out.print("str为空3\n");
        if(str == null || str == "")
            System.out.print("str为空4\n");

        long start = 0L;
        long end = 0L;
        start = System.currentTimeMillis();
        for (int i = 0; i < 9999; i++) {
            if(str == null || "".equals(str));
                //System.out.print("str为空1\n");
        }
        end = System.currentTimeMillis();
        System.out.println("使用equals的时间是:" + (end - start) + "毫秒!");


        start = System.currentTimeMillis();
        for (int i = 0; i < 9999; i++) {
            if(str == null || str.length() <= 0);
                //System.out.print("str为空1\n");
        }
        end = System.currentTimeMillis();
        System.out.println("使用length的时间是:" + (end - start) + "毫秒!");

        start = System.currentTimeMillis();
        for (int i = 0; i < 9999; i++) {
            if(str == null || str.isEmpty());
                //System.out.print("str为空1\n");
        }
        end = System.currentTimeMillis();
        System.out.println("使用isEmpty的时间是:" + (end - start) + "毫秒!");


        start = System.currentTimeMillis();
        for (int i = 0; i < 9999; i++) {
            if(str == null || str == "");
                //System.out.print("str为空1\n");
        }
        end = System.currentTimeMillis();
        System.out.println("直接判断的时间是:" + (end - start) + "毫秒!");

    }
}

执行结果:
使用equals的时间是:4毫秒!
使用length的时间是:0毫秒!
使用isEmpty的时间是:1毫秒!
直接判断的时间是:0毫秒!

//注意：
//1.四个方式的效率2，3，4的效率差不多，1相对慢一点，这个几种方式我试过循环1000次来比较，效率其实差不多，2，4方式最优
//2. 注意判断字符串是否为空，必须先判断null，否则用length或者equals判断会抛异常，java.lang.NullPointerException.
//3.对List、Array等进行操作时（特别是判断size、length时）先进行空引用判断

至于为啥出现效率的差异，下面附上JDK源码就非常清晰了。
   /**
     * Compares this string to the specified object.  The result is {
  
  @code
     * true} if and only if the argument is not {
  
  @code null} and is a {
  
  @code
     * String} object that represents the same sequence of characters as this
     * object.
     *
     * @param  anObject
     *         The object to compare this {
  
  @code String} against
     *
     * @return  {
  
  @code true} if the given object represents a {
  
  @code String}
     *          equivalent to this string, {
  
  @code false} otherwise
     *
     * @see  #compareTo(String)
     * @see  #equalsIgnoreCase(String)
     */
    public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String anotherString = (String)anObject;
        int n = count;
        if (n == anotherString.count) {
        char v1[] = value;
        char v2[] = anotherString.value;
        int i = offset;
        int j = anotherString.offset;
        while (n-- != 0) {
            if (v1[i++] != v2[j++])
            return false;
        }
        return true;
        }
    }
    return false;
    }

    /**
     * Returns <tt>true</tt> if, and only if, {
  
  @link #length()} is <tt>0</tt>.
     *
     * @return <tt>true</tt> if {
  
  @link #length()} is <tt>0</tt>, otherwise
     * <tt>false</tt>
     *
     * @since 1.6
     */
    public boolean isEmpty() {
    return count == 0;
    }
//1.isEmpty方法和length其实实现的原理是相同的，jdk 1.6版本之后建议用这个比较。
//2. equals 实现就相对复杂