import util.StopWatch;

/**
 * 循環(huán)優(yōu)化：
 * 除了本例中將循環(huán)不變量移出循環(huán)外，還有將忙循環(huán)放在外層
 * @author jxqlovejava
 *
 */
public class LoopOptimization {
    
    public int size() {
        try {
            Thread.sleep(200);   // 模擬耗時(shí)操作
        }
        catch(InterruptedException ie) {
            
        }
        
        return 10;
    }
    
    public void slowLoop() {
        StopWatch sw = new StopWatch("slowLoop");
        sw.start();
        
        for(int i = 0; i < size(); i++);
        
        sw.end();
        sw.printEclapseDetail();
    }
    
    public void optimizeLoop() {
        StopWatch sw = new StopWatch("optimizeLoop");
        sw.start();
        
        // 將循環(huán)不變量移出循環(huán)
        for(int i = 0, stop = size(); i < stop; i++);
        
        sw.end();
        sw.printEclapseDetail();
    }
    
    public static void main(String[] args) {
        LoopOptimization loopOptimization = new LoopOptimization();
        loopOptimization.slowLoop();
        loopOptimization.optimizeLoop();
    }

}

slowLoop任務(wù)耗時(shí)（毫秒）：2204
optimizeLoop任務(wù)耗時(shí)（毫秒）：211

import util.StopWatch;

/**
 * 數(shù)組索引訪問優(yōu)化，尤其針對(duì)多維數(shù)組
 * 這條優(yōu)化技巧對(duì)時(shí)間性能提升不太明顯，而且可能降低代碼可讀性
 * @author jxqlovejava
 *
 */
public class ArrayIndexAccessOptimization {
    
    private static final int m = 9;   // 9行
    private static final int n = 9;   // 9列
    private static final int[][] array = {
        { 1,  2,  3,  4,  5,  6,  7,   8, 9  },
        { 11, 12, 13, 14, 15, 16, 17, 18, 19 },
        { 21, 22, 23, 24, 25, 26, 27, 28, 29 },
        { 31, 32, 33, 34, 35, 36, 37, 38, 39 },
        { 41, 42, 43, 44, 45, 46, 47, 48, 49 },
        { 51, 52, 53, 54, 55, 56, 57, 58, 59 },
        { 61, 62, 63, 64, 65, 66, 67, 68, 69 },
        { 71, 72, 73, 74, 75, 76, 77, 78, 79 },
        { 81, 82, 83, 84, 85, 86, 87, 88, 89 },
        { 91, 92, 93, 94, 95, 96, 97, 98, 99 }
    };   // 二維數(shù)組
    

    public void slowArrayAccess() {
        StopWatch sw = new StopWatch("slowArrayAccess");
        sw.start();
        
        for(int k = 0; k < 10000000; k++) {
            int[] rowSum = new int[m];
            for(int i = 0; i < m; i++) {
                for(int j = 0; j < n; j++) {
                    rowSum[i] += array[i][j];
                }
            }
        }

        sw.end();
        sw.printEclapseDetail();
    }
    
    public void optimizeArrayAccess() {
        StopWatch sw = new StopWatch("optimizeArrayAccess");
        sw.start();
        
        for(int k = 0; k < 10000000; k++) {
            int[] rowSum = new int[n];
            for(int i = 0; i < m; i++) {
                int[] arrI = array[i];
                int sum = 0;
                for(int j = 0; j < n; j++) {
                    sum += arrI[j];
                }
                rowSum[i] = sum;
            }
        }
        
        sw.end();
        sw.printEclapseDetail();
    }
    
    public static void main(String[] args) {
        ArrayIndexAccessOptimization arrayIndexAccessOpt = new ArrayIndexAccessOptimization();
        arrayIndexAccessOpt.slowArrayAccess();
        arrayIndexAccessOpt.optimizeArrayAccess();
    }
    
}

import util.StopWatch;

/**
 * 優(yōu)化效果不明顯
 * @author jxqlovejava
 *
 */
public class IfElseOptimization {
    
    public void slowIfElse() {
        StopWatch sw = new StopWatch("slowIfElse");
        sw.start();
        
        for(int k = 0; k < 2000000000; k++) {
            int i = 9;
            if(i == 0) { }
            else if(i == 1) { }
            else if(i == 2) { }
            else if(i == 3) { }
            else if(i == 4) { }
            else if(i == 5) { }
            else if(i == 6) { }
            else if(i == 7) { }
            else if(i == 8) { }
            else if(i == 9) { }
        }
        
        sw.end();
        sw.printEclapseDetail();
    }
    
    public void optimizeIfElse() {
        StopWatch sw = new StopWatch("optimizeIfElse");
        sw.start();
        
        for(int k = 0; k < 2000000000; k++) {
            int i = 9;
            switch(i) {
            case 0:
                break;
            case 1:
                break;
            case 2:
                break;
            case 3:
                break;
            case 4:
                break;
            case 5:
                break;
            case 6:
                break;
            case 7:
                break;
            case 8:
                break;
            case 9:
                break;
            default:
            }
        }
        
        sw.end();
        sw.printEclapseDetail();
    }
    
    public static void main(String[] args) {
        IfElseOptimization ifElseOpt = new IfElseOptimization();
        ifElseOpt.slowIfElse();
        ifElseOpt.optimizeIfElse();
    }

}

class MyException extends Exception {
    
    /**
     * 
     */
    private static final long serialVersionUID = -1515205444433997458L;

    public Throwable fillInStackTrace() {
        return this;
    }
    
}

import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

import util.StopWatch;
 
public class ReadFileDemos {
    public static void main(String[] args) throws IOException {
        String filePath = "C:\\Users\\jxqlovejava\\workspace\\PerformanceOptimization\\test.txt";
        InputStream in = null;
        BufferedInputStream bis = null;
        File file = null;
        StopWatch sw = new StopWatch();
 
        sw.clear();
        sw.setTaskName("一次性讀取到字節(jié)數(shù)組+BufferedReader");
        sw.start();
        file = new File(filePath);
        in = new FileInputStream(filePath);
        BufferedReader br = new BufferedReader(new InputStreamReader(in));
        char[] charBuf = new char[(int) file.length()];
        br.read(charBuf);
        br.close();
        in.close();
        sw.end();
        sw.printEclapseDetail();
        
        sw.clear();
        sw.setTaskName("一次性讀取到字節(jié)數(shù)組");
        sw.start();
        in = new FileInputStream(filePath);
        byte[] buf = new byte[in.available()];
        in.read(buf);// read(byte[] buf)方法重載
        in.close();
        for (byte c : buf) {
        }
        sw.end();
        sw.printEclapseDetail();

        sw.clear();
        sw.setTaskName("BufferedInputStream逐字節(jié)讀取");
        sw.start();
        in = new FileInputStream(filePath);
        bis = new BufferedInputStream(in);
        int b;
        while ((b = bis.read()) != -1);
        in.close();
        bis.close();
        sw.end();
        sw.printEclapseDetail();

        sw.clear();
        sw.setTaskName("BufferedInputStream+DataInputStream分批讀取到字節(jié)數(shù)組");
        sw.start();
        in = new FileInputStream(filePath);
        bis = new BufferedInputStream(in);
        DataInputStream dis = new DataInputStream(bis);
        byte[] buf2 = new byte[1024*4]; // 4k per buffer
        int len = -1;
        StringBuffer sb = new StringBuffer();
        while((len=dis.read(buf2)) != -1 ) {
            // response.getOutputStream().write(b, 0, len);
            sb.append(new String(buf2));
        }
        dis.close();
        bis.close();
        in.close();
        sw.end();
        sw.printEclapseDetail();

        sw.clear();
        sw.setTaskName("FileInputStream逐字節(jié)讀取");
        sw.start();
        in = new FileInputStream(filePath);
        int c;
        while ((c = in.read()) != -1);
        in.close();
        sw.end();
        sw.printEclapseDetail();
    }
}

一次性讀取到字節(jié)數(shù)組+BufferedReader任務(wù)耗時(shí)（毫秒）：121
一次性讀取到字節(jié)數(shù)組任務(wù)耗時(shí)（毫秒）：23
BufferedInputStream逐字節(jié)讀取任務(wù)耗時(shí)（毫秒）：408
BufferedInputStream+DataInputStream分批讀取到字節(jié)數(shù)組任務(wù)耗時(shí)（毫秒）：147
FileInputStream逐字節(jié)讀取任務(wù)耗時(shí)（毫秒）：38122

/**
 * 秒表類，用于計(jì)算執(zhí)行時(shí)間
 * 注意該類是非線程安全的
 * @author jxqlovejava
 *
 */
public class StopWatch {
    
    private static final String DEFAULT_TASK_NAME = "defaultTask";
    private String taskName;
    private long start, end;
    private boolean hasStarted, hasEnded;
    
    // 時(shí)間單位枚舉：毫秒、秒和分鐘
    public enum TimeUnit { MILLI, SECOND, MINUTE  }
    
    public StopWatch() {
        this(DEFAULT_TASK_NAME);
    }
    
    public StopWatch(String taskName) {
        this.taskName = StringUtil.isEmpty(taskName) ? DEFAULT_TASK_NAME : taskName;
    }
    
    public void start() {
        start = System.currentTimeMillis();
        hasStarted = true;
    }
    
    public void end() {
        if(!hasStarted) {
            throw new IllegalOperationException("調(diào)用StopWatch的end()方法之前請(qǐng)先調(diào)用start()方法");
        }
        end = System.currentTimeMillis();
        hasEnded = true;
    }
    
    public void clear() {
        this.start = 0;
        this.end = 0;
        
        this.hasStarted = false;
        this.hasEnded = false;
    }
    
    /**
     * 獲取總耗時(shí)，單位為毫秒
     * @return 消耗的時(shí)間，單位為毫秒
     */
    public long getEclapsedMillis() {
        if(!hasEnded) {
            throw new IllegalOperationException("請(qǐng)先調(diào)用end()方法");
        }
        
        return (end-start);
    }
    
    /**
     * 獲取總耗時(shí)，單位為秒
     * @return 消耗的時(shí)間，單位為秒
     */
    public long getElapsedSeconds() {
        return this.getEclapsedMillis() / 1000;
    }
    
    /**
     * 獲取總耗時(shí)，單位為分鐘
     * @return 消耗的時(shí)間，單位為分鐘
     */
    public long getElapsedMinutes() {
        return this.getEclapsedMillis() / (1000*60);
    }
    
    public void setTaskName(String taskName) {
        this.taskName = StringUtil.isEmpty(taskName) ? DEFAULT_TASK_NAME : taskName;
    }
    
    public String getTaskName() {
        return this.taskName;
    }
    
    /**
     * 輸出任務(wù)耗時(shí)情況，單位默認(rèn)為毫秒
     */
    public void printEclapseDetail() {
        this.printEclapseDetail(TimeUnit.MILLI);
    }
    
    /**
     * 輸出任務(wù)耗時(shí)情況，可以指定毫秒、秒和分鐘三種時(shí)間單位
     * @param timeUnit 時(shí)間單位
     */
    public void printEclapseDetail(TimeUnit timeUnit) {
        switch(timeUnit) {
        case MILLI:
            System.out.println(this.getTaskName() + "任務(wù)耗時(shí)（毫秒）：" + this.getEclapsedMillis());
            break;
        case SECOND:
            System.out.println(this.getTaskName() + "任務(wù)耗時(shí)（秒）：" + this.getElapsedSeconds());
            break;
        case MINUTE:
            System.out.println(this.getTaskName() + "任務(wù)耗時(shí)（分鐘）：" + this.getElapsedMinutes());
            break;
        default:
            System.out.println(this.getTaskName() + "任務(wù)耗時(shí)（毫秒）：" + this.getEclapsedMillis());
        }
    }

}