import java.math.BigDecimal;import java.util.concurrent.CountDownLatch;import java.util.concurrent.atomic.AtomicInteger;import org.junit.Test;/** * 常见错误代码注意点 * @author Yuanqy * @time 2015年5月10日 9:23:00 */public class ExceptionTest {    /**     * 用例1：入箱与拆箱 
     * Integer、Short、Byte、Character、Long这几个包装类的valueOf方法的实现是类似的：会有缓存cache.high,cache.low；
     * Double、Float的valueOf方法的实现是类似的：要么直接赋值，要么通过String转。 
     * Boolean的valueOf方法的实现是个三目运算，形如` return (b ? TRUE : FALSE);     */    @Test    public void test1() {        System.out.println("==test1=======================================");//        //JDK,源代码：//        public static Integer valueOf(int i) {//            assert IntegerCache.high >= 127;//            if (i >= IntegerCache.low && i <= IntegerCache.high)//                return IntegerCache.cache[i + (-IntegerCache.low)];//            return new Integer(i);//        }        Integer i1 = 128;        Integer i2 = 128;        Integer i3 = 127;        Integer i4 = 127;        System.out.println("i1 == i2:" + (i1 == i2));// false        System.out.println("i3 == i4:" + (i3 == i4));// true    }    /**     * 用例2：入箱与拆箱 
     * 在java代码中，很多场景会用到装箱和拆箱操作，稍不留神，就会掉坑里。
     * 保证不出异常，就在运算过程中用同种类型。装箱就全部装箱。不装箱就全部不装箱。     */    @Test    public void test2() {        class TestBean {            private Integer test;            public Integer getTest() {                return test;            }            public void setTest(Integer test) {                this.test = test;            }        }        try {            System.out.println("==test2=======================================");            TestBean bean = new TestBean();            bean.setTest(bean != null ? bean.getTest() : new Integer(1)); // 这行正常            bean.setTest(bean != null ? bean.getTest() : 1);// 这行异常【很经典的异常】，原因是三元表达式的执行过程和 int/Integer 的入箱与拆箱 过程照成的。        } catch (Exception e) {            e.printStackTrace();            throw e;        }    }    /**     * 用例3：注意数据溢出     */    @Test    public void test3() {        System.out.println("==test3=======================================");        int s = 1;        for (int i = 10; i <= 49; i++) {            s = s * i;            System.out.println(s + "==" + Integer.toBinaryString(s));        }        System.out.println("结果是：" + s);// 结果是：0    }    /**     * 用例4：不要相信 get/Select，要保证操作原子性     * 并且：基本数据类型的 ++ ，-- 运算符 也非线程安全，是非原子性的。     * 计数操作，使用AtomicXX类操作     * 还有：Thread、Runnable、Callable、Future、FutureTask，这些多线程操作都有线程安全问题，需要特殊处理。     * 【网上误人子弟的真JB多，特别是IO，多线程的文章】。     * @throws InterruptedException     */    @Test    public void test4() throws InterruptedException {        System.out.println("==test4=======================================");        int sum = 100;        final CountDownLatch latch = new CountDownLatch(sum);        Thread t = new Thread() {            private AtomicInteger count = new AtomicInteger(0);// 计数不能使用int做++count            private BigDecimal money = BigDecimal.ZERO;            public BigDecimal getMoney() {                return money;            }            public void setMoney(BigDecimal money) {                this.money = money;            }            @Override            public void run() {                BigDecimal curM = BigDecimal.ZERO;                // synchronized (curM) {//加同步块或读写锁                try {                    curM = getMoney();                    curM = curM.add(BigDecimal.ONE);                    setMoney(curM);                    System.out.println(count.addAndGet(1) + ":getMoney=" + getMoney().toPlainString());                    latch.countDown();                } catch (Exception e) {                    e.printStackTrace();                }                // }            }        };        // t只实例一次        for (int i = 0; i < sum; i++) {            new Thread(t).start();        }        latch.await();        System.out.println("结果：执行了100次加操作，结果却不是100。");    }    /**     * 用例5：涉及到小数操作，不允许出现float double浮点数类型     * 特别是金融场景     */    @Test    public void test5() {        System.out.println("==test5=======================================");        System.out.println(0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1);        System.out.println(0.1 * 10);    }    /**     * 浮点数操作，最好使用BigDecimal 或 String     */    @Test    public void test6() {        System.out.println("==test6=======================================");        Double d=123456789.98D;                System.out.println(new BigDecimal(d).toString());// 错        System.out.println(new Double(d).toString());// 错        System.out.println(new BigDecimal(new Double(d).toString()).toString());// 对        System.out.println(new BigDecimal(d).toPlainString());// 错    }    /**     * 代码都是假随机。     * 可以预测的，甚至可以输出想要的字符。     */    @Test    public void test7() {        System.out.println("==test7=======================================");        class RandomBuild {            public String getStr(long seed) {                Random ran = new Random(seed);                StringBuilder sb = new StringBuilder();                while (true) {                    int k = ran.nextInt(27);                    System.out.println("char:" + k + ",number:" + k);                    if (k == 0) break;                    k += 96;                    sb.append((char) k);                }                return sb.toString();            }        }        RandomBuild rb = new RandomBuild();        System.out.println(rb.getStr(-229985452) + " " + rb.getStr(-147909649));    }}

执行结果控制台：

==test1=======================================i1 == i2:falsei3 == i4:true==test2=======================================java.lang.NullPointerException	at ExceptionTest.test2(ExceptionTest.java:60)	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)	at java.lang.reflect.Method.invoke(Method.java:606)	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)	at org.junit.runners.ParentRunner.run(ParentRunner.java:309)	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)==test3=======================================10==1010110==11011101320==1010010100017160==100001100001000240240==1110101010011100003603600==110110111111001001000057657600==11011011111100100100000000980179200==111010011011000101100100000000463356416==11011100111100100001000000000213837312==1100101111101110011000000000-18221056==11111110111010011111100000000000-382642176==11101001001100010101100000000000171806720==1010001111011001000000000000-343412736==11101011100001111111000000000000348028928==10100101111101000000000000000110788608==110100110101000000000000000-1414463488==10101011101100010000000000000000464191488==11011101010110000000000000000112459776==110101101000000000000000000-1033633792==11000010011001000000000000000000-944242688==11000111101110000000000000000000793247744==101111010010000000000000000000-385875968==11101001000000000000000000000000150994944==1001000000000000000000000000838860800==110010000000000000000000000000-704643072==11010110000000000000000000000000402653184==110000000000000000000000000002013265920==1111000000000000000000000000000-805306368==11010000000000000000000000000000-1342177280==10110000000000000000000000000000-2147483648==10000000000000000000000000000000-2147483648==100000000000000000000000000000000==00==00==00==00==00==00==00==0结果是：0==test4=======================================1:getMoney=12:getMoney=23:getMoney=34:getMoney=45:getMoney=56:getMoney=67:getMoney=78:getMoney=89:getMoney=910:getMoney=1011:getMoney=1112:getMoney=1213:getMoney=1314:getMoney=1415:getMoney=1516:getMoney=1617:getMoney=1718:getMoney=1819:getMoney=1920:getMoney=2021:getMoney=2122:getMoney=2223:getMoney=2224:getMoney=2325:getMoney=2426:getMoney=2527:getMoney=2628:getMoney=2729:getMoney=2830:getMoney=2931:getMoney=3032:getMoney=3133:getMoney=3234:getMoney=3235:getMoney=3336:getMoney=3337:getMoney=3438:getMoney=3539:getMoney=3640:getMoney=3741:getMoney=3842:getMoney=3944:getMoney=3943:getMoney=3847:getMoney=4249:getMoney=4446:getMoney=4151:getMoney=4653:getMoney=4845:getMoney=4054:getMoney=4957:getMoney=5255:getMoney=5060:getMoney=5561:getMoney=5662:getMoney=5752:getMoney=4750:getMoney=4564:getMoney=5948:getMoney=4367:getMoney=6168:getMoney=6269:getMoney=6371:getMoney=6566:getMoney=6072:getMoney=6674:getMoney=6865:getMoney=5975:getMoney=6976:getMoney=7077:getMoney=7163:getMoney=5879:getMoney=7380:getMoney=7459:getMoney=5481:getMoney=7582:getMoney=7658:getMoney=5384:getMoney=7885:getMoney=7956:getMoney=5183:getMoney=7778:getMoney=7289:getMoney=8373:getMoney=6791:getMoney=8470:getMoney=6499:getMoney=8598:getMoney=9096:getMoney=8997:getMoney=8995:getMoney=8794:getMoney=86100:getMoney=8593:getMoney=8592:getMoney=8590:getMoney=8388:getMoney=8287:getMoney=8186:getMoney=80结果：执行了100次加操作，结果却不是100。==test5=======================================0.99999999999999991.0==test6=======================================123456789.980000004172325134277343751.2345678998E8123456789.98123456789.98000000417232513427734375==test7=======================================char:8,number:8char:5,number:5char:12,number:12char:12,number:12char:15,number:15char:0,number:0char:23,number:23char:15,number:15char:18,number:18char:12,number:12char:4,number:4char:0,number:0hello world