J2ME循环与String优化设计

循环的最佳化
通常我们撰写循环时，习惯让计数变量从0 开始，然后累加。但是，只要做一点小小的更改，就可以让程序跑的更快，范例程序如下:

//PerfTest.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class PerfTest extends MIDlet {
    public PerfTest() {
    }

    public void startApp() {
        long start, end;
        start = System.currentTimeMillis();
        for (int i = 0; i < 9000000; i++) {
            ;
        }
        end = System.currentTimeMillis();
        System.out.println("Normal loop : " + (end - start) + " milliseconds");
        start = System.currentTimeMillis();
        for (int i = 8999999; i >= 0; i--) {
            ;
        }
        end = System.currentTimeMillis();
        System.out.println("Optimized loop : " + (end - start) +
                           " milliseconds");
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
    }
}

//StringBufferTest.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class StringBufferTest extends MIDlet {
    public StringBufferTest() {
    }

    public void startApp() {
        Runtime rt = Runtime.getRuntime();
        long diffstart = rt.totalMemory() - rt.freeMemory();
        System.out.println("Memory used Start:" + diffstart);
        //注意，底下的程序代码没有任何意义，纯粹要展示String 有多耗内存
        String sum = "";
        for (int i = 0; i < 5000; i++) {
            sum = sum + "+" + i;
        }
        long diffend = rt.totalMemory() - rt.freeMemory();
        System.out.println("Memory used End:" + diffend);
        System.out.println("Memory used after operation:" +
                           (diffend - diffstart));
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
    }
}

//StringBufferTest.java
import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

public class StringBufferTest extends MIDlet {
    public StringBufferTest() {
    }

    public void startApp() {
        Runtime rt = Runtime.getRuntime();
        long diffstart = rt.totalMemory() - rt.freeMemory();
        System.out.println("Memory used Start:" + diffstart);
        //注意，底下的程序代码没有任何意义，纯粹要展示StringBuffer
        StringBuffer sum = new StringBuffer("");
        for (int i = 0; i < 5000; i++) {
            sum = sum.append("+").append(i);
        }
        long diffend = rt.totalMemory() - rt.freeMemory();
        System.out.println("Memory used End:" + diffend);
        System.out.println("Memory used after operation:" +
                           (diffend - diffstart));
    }

    public void pauseApp() {
    }

    public void destroyApp(boolean unconditional) {
    }
}