經常需要判斷一個字串變數是否為空，今天特地做了個小小的測試

StringUtils.java:

package com.yx.equipment_collection.utils;

import android.annotation.SuppressLint;
import android.text.TextUtils;
import android.util.Log;

/**
 * 
 * 此類描述的是: String幫助類
 * 
 * @author: CS YX
 * @version:1.0
 * @date:2014-10-21 下午2:47:08
 */
public class StringUtils {
	private static final String TAG = "StringUtils";
	private static int count = 100000000;

	public static void checkEmpty1(String str) {
		long start = System.currentTimeMillis();
		for (int i = 0; i < count; i++) {
			if (str == null || str.length() < 1) {
			}
		}
		long end = System.currentTimeMillis();
		Log.i(TAG, "checkEmpty1 --- " + (end - start));
	}

	@SuppressLint("NewApi")
	public static void checkEmpty2(String str) {
		long start = System.currentTimeMillis();
		for (int i = 0; i < count; i++) {
			if (str == null || str.isEmpty()) {
			}
		}
		long end = System.currentTimeMillis();
		Log.i(TAG, "checkEmpty2 --- " + (end - start));
	}

	public static void checkEmpty3(String str) {
		long start = System.currentTimeMillis();
		for (int i = 0; i < count; i++) {
			if (str == null || str == "") {
			}
		}
		long end = System.currentTimeMillis();
		Log.i(TAG, "checkEmpty3 --- " + (end - start));
	}

	public static void checkEmpty4(String str) {
		long start = System.currentTimeMillis();
		for (int i = 0; i < count; i++) {
			if (str == null || str.equals("")) {
			}
		}
		long end = System.currentTimeMillis();
		Log.i(TAG, "checkEmpty4 --- " + (end - start));

	}

	public static void checkEmpty5(String str) {
		long start = System.currentTimeMillis();
		for (int i = 0; i < count; i++) {
			if (str == null || TextUtils.isEmpty(str)) {
			}
		}
		long end = System.currentTimeMillis();
		Log.i(TAG, "checkEmpty5 --- " + (end - start));

	}

	public static void checkEmpty11(String str) {
		long start = System.currentTimeMillis();
		for (int i = 0; i < count; i++) {
			if (str != null && str.length() > 0) {
			}
		}
		long end = System.currentTimeMillis();
		Log.i(TAG, "checkEmpty11 --- " + (end - start));
	}

	@SuppressLint("NewApi")
	public static void checkEmpty22(String str) {
		long start = System.currentTimeMillis();
		for (int i = 0; i < count; i++) {
			if (str != null && !str.isEmpty()) {
			}
		}
		long end = System.currentTimeMillis();
		Log.i(TAG, "checkEmpty22 --- " + (end - start));
	}

	public static void checkEmpty33(String str) {
		long start = System.currentTimeMillis();
		for (int i = 0; i < count; i++) {
			if (str != null && str != "") {
			}
		}
		long end = System.currentTimeMillis();
		Log.i(TAG, "checkEmpty33 --- " + (end - start));
	}

	public static void checkEmpty44(String str) {
		long start = System.currentTimeMillis();
		for (int i = 0; i < count; i++) {
			if (str != null && !str.equals("")) {
			}
		}
		long end = System.currentTimeMillis();
		Log.i(TAG, "checkEmpty44 --- " + (end - start));

	}

	public static void checkEmpty55(String str) {
		long start = System.currentTimeMillis();
		for (int i = 0; i < count; i++) {
			if (str != null && !TextUtils.isEmpty(str)) {
			}
		}
		long end = System.currentTimeMillis();
		Log.i(TAG, "checkEmpty55 --- " + (end - start));

	}

}
 

測試為空如下：test

	public void test() {
		String str = "";
		Log.i("test", "str=\"\"---");
		StringUtils.checkEmpty1(str);
		StringUtils.checkEmpty2(str);
		StringUtils.checkEmpty3(str);
		StringUtils.checkEmpty4(str);
		StringUtils.checkEmpty5(str);
		str = null;
		Log.i("test", "str=null---");
		StringUtils.checkEmpty1(str);
		StringUtils.checkEmpty2(str);
		StringUtils.checkEmpty3(str);
		StringUtils.checkEmpty4(str);
		StringUtils.checkEmpty5(str);
		str = "null";
		Log.i("test", "str=\"null\"---");
		StringUtils.checkEmpty1(str);
		StringUtils.checkEmpty2(str);
		StringUtils.checkEmpty3(str);
		StringUtils.checkEmpty4(str);
		StringUtils.checkEmpty5(str);
		str = new String();
		Log.i("test", "str=new String()---");
		StringUtils.checkEmpty1(str);
		StringUtils.checkEmpty2(str);
		StringUtils.checkEmpty3(str);
		StringUtils.checkEmpty4(str);
		StringUtils.checkEmpty5(str);

	}
 

測試結果輸入如下圖：

由此圖可以看出方法3(str == "")用時是最少的；其次就是方法1(str.length() < 1)和方法2(str.isEmpty()) ;

方法4(str.equals(""))耗時較多;方法5(TextUtils.isEmpty(str))最耗時

 測試非空如下：test

	public void test1() {
		String str = "";
		Log.i("test", "str=\"\"---");
		StringUtils.checkEmpty11(str);
		StringUtils.checkEmpty22(str);
		StringUtils.checkEmpty33(str);
		StringUtils.checkEmpty44(str);
		StringUtils.checkEmpty55(str);
		str = null;
		Log.i("test", "str=null---");
		StringUtils.checkEmpty11(str);
		StringUtils.checkEmpty22(str);
		StringUtils.checkEmpty33(str);
		StringUtils.checkEmpty44(str);
		StringUtils.checkEmpty55(str);
		str = "null";
		Log.i("test", "str=\"null\"---");
		StringUtils.checkEmpty11(str);
		StringUtils.checkEmpty22(str);
		StringUtils.checkEmpty33(str);
		StringUtils.checkEmpty44(str);
		StringUtils.checkEmpty55(str);
		str = new String();
		Log.i("test", "str=new String()---");
		StringUtils.checkEmpty11(str);
		StringUtils.checkEmpty22(str);
		StringUtils.checkEmpty33(str);
		StringUtils.checkEmpty44(str);
		StringUtils.checkEmpty55(str);
	}
 

測試結果如下圖：

如上圖所示，首先是方法33(str != null && str != "")比較佔優勢；方法11(str != null && str.length() > 0)和方法22(str != null && !str.isEmpty())總體來說，不相上下；

方法44(str != null && !str.equals(str != null && !TextUtils.isEmpty(str)))較耗時；方法55()還是最耗時

也有人說，用‘==’判斷字串不準確，應該用‘equals’，個人覺得String一般都是直接=定義，想必沒有幾個人定義一個字串會new出來吧。

為什麼TextUtils.isEmpty()耗時最多，檢視原始碼原來isEmpty()已經判斷了‘==null’：

    /**
     * Returns true if the string is null or 0-length.
     * @param str the string to be examined
     * @return true if str is null or zero length
     */
    public static boolean isEmpty(CharSequence str) {
        if (str == null || str.length() == 0)
            return true;
        else
            return false;
    }

原始碼也是用.length()判斷的，如果你覺得‘==’不靠譜，推薦使用.length()方法判斷！

以上純屬個人見解......謝謝