有时候方法返回一个列表但是不想调用者改变列表内容。有三种方法可以实现不可变列表,通过调用JDK,Guava以及Apache Commons Collections相关API来实现。

1.JDK

 @Test
 public void jdkTest() {
     List<String> fruitsList = new ArrayList<String>(Arrays.asList("Apple", "Orange"));
     List<String> fruitsUnmodifiableList = Collections.unmodifiableList(fruitsList);
     fruitsUnmodifiableList.add("Pear");
 }

add方法会抛出java.lang.UnsupportedOperationException异常。

2.Apache Commons Collections

@Test
public void accTest() {
    List<String> fruitsList = new ArrayList<String>(Arrays.asList("Apple", "Orange"));
    List<String> fruitsUnmodifiableList = ListUtils.unmodifiableList(fruitsList);
    fruitsUnmodifiableList.add("Pear");
}

同样的调用add方法会抛异常,源码:

 public boolean add(Object object) {
    throw new UnsupportedOperationException();
  }

  

3.Guava

@Test
public void guavaTest() {
    List<String> fruitsList = new ArrayList<String>(Arrays.asList("Apple", "Orange"));
    List<String> fruitsUnmodifiableList = ImmutableList.copyOf(fruitsList);
    fruitsUnmodifiableList.add("Pear");
}

这个操作实际创建了原list的copy,不是原list的视图,Guava也提供构建器——返回强类型ImmutableList代替简单List,如下:

@Test
public void guavaTest2() {
    List<String> fruitsList = new ArrayList<String>(Arrays.asList("Apple", "Orange"));
    ImmutableList<Object> list = ImmutableList.builder().addAll(fruitsList).build();
    list.add("Pear");
}