常见的几种改善代码效率的技巧

1. 尽量在循环外使用try语句捕获异常

public class CatchTest {
    public static void main(String[] args) {
        int a = 0;
        long start = System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            try {
                a++;
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        System.out.println("innerCatch:" + (System.currentTimeMillis() - start));

        long start2 = System.currentTimeMillis();
        try {
            for (int i = 0; i < 1000000000; i++) {
                a++;
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println("outerCatch:" + (System.currentTimeMillis() - start2));
    }
}

Sout:
innerCatch:3
outerCatch:2

2. 能有局部变量的地方尽量不使用全局变量

public class LocalVariableTest {
    private static int a = 2;

    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            a++;
        }
        System.out.println("staticVariable:" + (System.currentTimeMillis() - start));

        int a = 0;
        long start2 = System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            a++;
        }
        System.out.println("localVariable:" + (System.currentTimeMillis() - start2));
    }


}
sout:
staticVariable:87
localVariable:3

3. 提取公共表达式

public class ExpressionTest {
    public static void main(String[] args) {
        double a = Math.random();
        double b = Math.random();
        double c = Math.random();
        double d = Math.random();
        double x, y;
        long start = System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            x = a * b * c / 3 * 4 * d;
            y = b * d * c / 3 * 4 * d;
        }
        System.out.println("staticVariable:" + (System.currentTimeMillis() - start));

        long start2 = System.currentTimeMillis();
        double z;
        for (int i = 0; i < 1000000000; i++) {
            z = c / 3 * 4 * d;
            x = a * b * z;
            y = b * d * z;
        }
        System.out.println("staticVariable:" + (System.currentTimeMillis() - start2));
    }
}
Sout:
staticVariable:7
staticVariable:2

4. 采用位运算代替普通运算

public class ByteCalcTest {
    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        int a = 3;
        for (int i = 0; i < 1000000000; i++) {
            a /= 2;
            a *= 2;
        }
        System.out.println("ordinal:" + (System.currentTimeMillis() - start));

        long start2 = System.currentTimeMillis();
        int b = 3;
        for (int i = 0; i < 1000000000; i++) {
            b <<= 1;
            b >>= 1;
        }
        System.out.println("bit:" + (System.currentTimeMillis() - start2));
    }
}
sout:
ordinal:1188
bit:3

5. 数组复制时采用System.arrayCopy()方法

public class ArrayTest {
    public static void main(String[] args) {
        int[] a = {1, 2, 3, 4};
        long start1 = System.currentTimeMillis();
        int[] b = new int[a.length];
        for (int i = 0; i < 1000000000; i++) {
            b = a.clone();
        }
        System.out.println("Object.clone():" + (System.currentTimeMillis() - start1));

        long start2 = System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            System.arraycopy(a, 0, b, 0, a.length);
        }
        System.out.println("System.arraycopy():" + (System.currentTimeMillis() - start2));

        long start3 = System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            b = Arrays.copyOf(a, a.length);
        }
        System.out.println("Array.copyOf():" + (System.currentTimeMillis() - start3));

        long start4 = System.currentTimeMillis();
        for (int i = 0; i < 1000000000; i++) {
            for (int j = 0; j < a.length; j++) {
                b[j] = a[j];
            }
        }
        System.out.println("for循环:" + (System.currentTimeMillis() - start4));
    }
}
sout:
Object.clone():8141
System.arraycopy():5326
Array.copyOf():7356
for循环:4260

但是，如果将数组a中的数据换成一个如下对象:

public class A {}

各数组复制方法时间损耗如下:

Object.clone():8018
System.arraycopy():6376
Array.copyOf():6894
for循环:7538

结论:当数组元素较简单时，各种数组复制方法的性能比较如下：

for循环>System.arraycopy()>Arrays.copyof()>Object.clone() 当数组元素较复杂时，各种数组复制方法的性能比较如下： System.arraycopy()>Arrays.copyof()>for循环>Object.clone()