equals--isempty--length

length  与 isempty 相似， null不可以使用，报空指针异常，

new string() 分配了空间，只是没有存储内容

“”空串，length=0

 

 

public static void main(String []ar){
		String a = null;
		String b = new String();
		String c = "";
		String d = "abcd";
		//NullPointerException
		System.out.println(a.isEmpty());
		System.out.println(a.length());
		//true 0 
		System.out.println(b.isEmpty());
		System.out.println(b.length());
		//true 0
		System.out.println(c.isEmpty());
		System.out.println(c.length());
		//false 4
		System.out.println(d.isEmpty());
		System.out.println(d.length());
	}

 

 

 

转;

判断String是否为空的小技巧

博客分类：

 
• java

 

             //比较 方法一，大多数人都这么比较

    String param = config.getInitParameter("log-level");
    if (param != null && !"".equals(param)) {
        final Level level = Log.getLevel(param);
        if (level != null) Log.lookup("org.zkoss").setLevel(level);
        else log.error("Unknown log-level: "+param);
    }
            //比较 方法二，（而我研究的源码基本都这么比较）
             String param = config.getInitParameter("log-level");
    if (param != null && param.length() > 0) {
        final Level level = Log.getLevel(param);
        if (level != null) Log.lookup("org.zkoss").setLevel(level);
        else log.error("Unknown log-level: "+param);
    }

// 下面我们看一下equals,length,isEmpty的源码

 //String中equals方法内部代码

 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;
   }

  //String.length()的源码

/**
    * Returns the length of this string.
    * The length is equal to the number of <a href="Character.html#unicode">Unicode
    * code units</a> in the string.
    *
    * @return  the length of the sequence of characters represented by this
    *          object.
    */
   public int length() {
       return count;
   }

   //String.isEmpty的源码
 /**
    * 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;
   }

 

仅从代码多少以及逻辑判断来看，length,isEmpty都非常简单，对于一般比较判断来说，

 

首先我猜测：length,isEmpty是相差无几的，equals方法就望尘莫及了，

 

 

 

下面做一个测试，证明我们的猜测

 

测试代码如下

 

 

private static final String sunflower = "sunflower";
    private static final int COUNT= 1000000000;

    public static void main(String[] args) {
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("开始计算...\n");

        Long begin = System.currentTimeMillis();
        for (int j = 0; j < COUNT; j++) {
            testEmptyByLength(sunflower);
        }
        calculateDuration("length test", begin);

        begin = System.currentTimeMillis();

        for (int j = 0; j < COUNT; j++) {
            testEmptyByEmpty(sunflower);
        }

        calculateDuration("empty test", begin);

        begin = System.currentTimeMillis();

        for (int j = 0; j < COUNT; j++) {
            testEmptyByEquals(sunflower);
        }
        calculateDuration("equals test", begin);
    }

    public static void calculateDuration(String method, long begin) {
        System.out.println(method + "\t:"
                + (System.currentTimeMillis() - begin) / 1000);
    }

    public static boolean testEmptyByLength(String str) {
        return str == null || str.length() < 1;
    }

    /**
     * empty方法是jdk1.6添加的方法，如果是早期版本不兼容
     *
     * @param str
     * @return
     */
    public static boolean testEmptyByEmpty(String str) {
        return str == null || str.isEmpty();
    }

    public static boolean testEmptyByEquals(String str) {
        return "".equals(str);
    }

 

 

输出结果：

 

 

 

 

开始计算（s）

 

length test:0

empty test:0

equlas test:8

 

 

 

 

结论：

 

String.isEmpty()方法是jdk1.6中添加的，其方法体和length()方法一样，从性能和兼容性，length()更好，

 

所以如果对字符串非严格判断是否为空，建议使用String.length>0判断,如果严格判断是否全部为" "字符，就要自己写或者用common lang StringUtils.isBlank了

 

isBlank方法代码补充：

 

 

 

/**
   * <p>Checks if a String is whitespace, empty ("") or null.</p>
   *
   * <pre>
   * StringUtils.isBlank(null)      = true
   * StringUtils.isBlank("")        = true
   * StringUtils.isBlank(" ")       = true
   * StringUtils.isBlank("bob")     = false
   * StringUtils.isBlank("  bob  ") = false
   * </pre>
   *
   * @param str  the String to check, may be null
   * @return <code>true</code> if the String is null, empty or whitespace
   * @since 2.0
   */
  public static boolean isBlank(String str) {
      int strLen;
      if (str == null || (strLen = str.length()) == 0) {
          return true;
      }
      for (int i = 0; i < strLen; i++) {
          if ((Character.isWhitespace(str.charAt(i)) == false)) {
              return false;
          }
      }
      return true;
  }

 

 

 

注：本人不是性能主义者，而是追求更好的方法！！