Java常见错误合集-白红宇

Java常见错误合集

阅读量：423 次

发布时间：2019-03-06

本文共 3106 字，大约阅读时间需要 10 分钟。

null 转基本类型

@Data@NoArgsConstructor@AllArgsConstructorstatic class OneClass {    private Double value;}@Testpublic void nullToPrimitive() {    OneClass oneClass = new OneClass(1.0);    double value = oneClass.getValue();    System.out.println(value);    OneClass oneClass2 = new OneClass();    value = oneClass2.getValue();    System.out.println(value);}null指针异常

String.format中输出%

@Testpublic void formatTest() {    String str = "the fee rage is %.2f%%";    System.out.println(String.format(str, 1.2));}the fee rage is 1.20%

google cache null 指针

@Testpublic void nullTest1() {    LoadingCache
   
     cache = CacheBuilder.newBuilder().expireAfterWrite(1, TimeUnit.HOURS).build(            new CacheLoader
    
     () {                @Override                public String load(String key) throws Exception {                    return null;                }            });    try {        System.out.println(cache.get("str"));    } catch (Exception e) {        log.error("nullTest1 {}", e.getMessage(), e);    }}报错：nullTest1 CacheLoader returned null for key str.

修改后

@Testpublic void nullTest2() {    LoadingCache
   
    > cache =            CacheBuilder.newBuilder().expireAfterWrite(1, TimeUnit.HOURS).build(                    new CacheLoader
    
     >() {                        @Override                        public Optional
     
       load(String key) throws Exception {                            return Optional.ofNullable(null);                        }                    });    try {        System.out.println(cache.get("str").isPresent() ? cache.get("str") : null);    } catch (Exception e) {        log.error("nullTest2 {}", e.getMessage(), e);    }}输出：null

double相加精度问题

@Testpublic void doubleAdd() {    System.out.println(0.3 + 0.3 + 0.3 + 0.1);    System.out.println(Double.compare(1.0, 0.3 + 0.3 + 0.3 + 0.1) == 0);    BigDecimal bigDecimal1 = BigDecimal.valueOf(0.3)            .add(BigDecimal.valueOf(0.3))            .add(BigDecimal.valueOf(0.3))            .add(BigDecimal.valueOf(0.1));    BigDecimal bigDecimal2 = BigDecimal.valueOf(1.0);    System.out.println(bigDecimal1.compareTo(bigDecimal2) == 0);}输出：0.9999999999999999falsetrue

contains 参数是object 类型不同

@Testpublic void containsTest() {    Set
   
     set = Sets.newHashSet((byte) 1, (byte) 2, (byte) 3);    System.out.println(set.contains(1));    System.out.println(set.contains((byte) 1));}输出：falsetrue

Arrays.asList

Integer[] integers = {0, 1, 2};List
   
     list = Arrays.asList(integers);System.out.println(list);integers[0] += 1;System.out.println(list);输出：[0, 1, 2][1, 1, 2]Arrays.asList 产生的ArrayList，它内部的数组直接沿用了这个数组，所以数组的修改，ArrayList也会相应的修改这个list不支持add和remove，支持set和get源码如下public static 
    
      List
     
       asList(T... a) {    return new ArrayList<>(a);}private static class ArrayList
      
        extends AbstractList
       
            implements RandomAccess, java.io.Serializable{    private static final long serialVersionUID = -2764017481108945198L;    private final E[] a;    ArrayList(E[] array) {        a = Objects.requireNonNull(array);    }}