String、StringBuilder、StringBuffer對比

參考：http://swiftlet.net/archives/1694

http://www.cnblogs.com/springcsc/archive/2009/12/03/1616326.html

 

String　　字符串（character strings），不是線程安全的

StringBuilder　　非線程安全的字符串序列（A mutable sequence of characters）

StringBuffer　　線程安全的字符串序列（A thread-safe, mutable sequence of characters）

一、初始化方式

　　String　　　　

String str="abc";
String str1=new String("abc");

　　這兩種初始化的方式的差異是：後者產生了兩個對象，內存浪費。

　　StringBuffer

StringBuffer sb1=new StringBuffer("abc");
StringBuffer sbf1=new StringBuffer();
sbf1.append("abc");            

　　從源碼上沒看出來這兩種的明顯差別。有老司機明白的能夠指點一二。

　　StringBuilder

　　和StringBuffer的初始化方式是同樣的。

二、性能比較

　　測試代碼　　

package cn.simple.src_study;

public class StringStudy {
    public static void main(String[] args) {
        long strStart = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {

            String str = "abc";
        }
        long strEnd = System.currentTimeMillis();
        System.out.println("str=\"abc\"; 執行10000次耗時：" + (strEnd - strStart));

        long strStart1 = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {

            String str1 = new String("abc");
        }
        long strEnd1 = System.currentTimeMillis();
        System.out.println("str=new String(\"abc\"); 執行10000次耗時：" + (strEnd1 - strStart1));

        long sbfStart = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {

            StringBuffer sbf = new StringBuffer("abc");
        }
        long sbfEnd = System.currentTimeMillis();
        System.out.println("sbf=new StringBuffer(\"abc\"); 執行10000次耗時：" + (sbfEnd - sbfStart));

        long sbfStart1 = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            StringBuffer sbf1 = new StringBuffer();
            sbf1.append("abc");
        }
        long sbfEnd1 = System.currentTimeMillis();
        System.out.println("sbf.append(\"abc\"); 執行10000次耗時：" + (sbfEnd1 - sbfStart1));

        long sbdStart = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            StringBuilder sbd = new StringBuilder("abc");
        }
        long sbdEnd = System.currentTimeMillis();
        System.out.println("sbd=new StringBuilder(\"abc\"); 執行10000次耗時：" + (sbdEnd - sbdStart));

        long sbdStart1 = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            StringBuilder sbd = new StringBuilder();
            sbd.append("abc");
        }
        long sbdEnd1 = System.currentTimeMillis();
        System.out.println("sbd.append(\"abc\"); 執行10000次耗時：" + (sbdEnd1 - sbdStart1));

    }
}

View Code

　　結果截圖

　　從測試結果能夠看出：若是值是固定的直接賦值給字符串，效率最高。

　　若是字符串存在拼接過程呢？

　　測試代碼：

package cn.simple.src_study;

public class StringStudy2 {
    public static void main(String[] args) {
        long strStart = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {

            String str = "abc";
            str+="def";
        }
        long strEnd = System.currentTimeMillis();
        System.out.println("str=\"abc\"; 執行10000次耗時：" + (strEnd - strStart));

        long strStart1 = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {

            String str1 = new String("abc");
            str1+="edf";
        }
        long strEnd1 = System.currentTimeMillis();
        System.out.println("str=new String(\"abc\"); 執行10000次耗時：" + (strEnd1 - strStart1));

        long sbfStart = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {

            StringBuffer sbf = new StringBuffer("abc");
            sbf.append("edf");
        }
        long sbfEnd = System.currentTimeMillis();
        System.out.println("sbf=new StringBuffer(\"abc\"); 執行10000次耗時：" + (sbfEnd - sbfStart));

        long sbfStart1 = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            StringBuffer sbf1 = new StringBuffer();
            sbf1.append("abc");
            sbf1.append("edf");
        }
        long sbfEnd1 = System.currentTimeMillis();
        System.out.println("sbf.append(\"abc\"); 執行10000次耗時：" + (sbfEnd1 - sbfStart1));

        long sbdStart = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            StringBuilder sbd = new StringBuilder("abc");
            sbd.append("edf");
        }
        long sbdEnd = System.currentTimeMillis();
        System.out.println("sbd=new StringBuilder(\"abc\"); 執行10000次耗時：" + (sbdEnd - sbdStart));

        long sbdStart1 = System.currentTimeMillis();
        for (int i = 0; i < 10000; i++) {
            StringBuilder sbd = new StringBuilder();
            sbd.append("abc");
            sbd.append("edf");
        }
        long sbdEnd1 = System.currentTimeMillis();
        System.out.println("sbd.append(\"abc\"); 執行10000次耗時：" + (sbdEnd1 - sbdStart1));

    }
}

View Code

　　測試結果：

　　從測試結果能夠看出：存在字符串拼接的狀況下性能StringBuilder>StringBuffer>String