要点Java20 java.util.Collections

java.util.Collections 集合帮助类

演示样例程序(JUnit演示)

排序

    @Test
    public void testSort() {
        List<Integer> demoList = new ArrayList<Integer>(Arrays.asList(3, 2, 1));
        assertEquals(3, demoList.get(0).intValue());

        //public static <T extends Comparable<?

super T>> void sort(List<T> list)
        Collections.sort(demoList);

        assertEquals(1, demoList.get(0).intValue());

        //public static void shuffle(List<?

> list) // 随机排序
        //public static void reverse(List<?

> list) // 反序
    }

查找

    @Test
    public void testBinarySearch() {
        List<Integer> demoList = new ArrayList<Integer>(Arrays.asList(3, 2, 1));
        //必须先排序
        Collections.sort(demoList);
        assertEquals(1, demoList.get(0).intValue());

        //二分查找位置
        //public static <T> int binarySearch(List<? extends Comparable<?

super T>> list, T key)
        assertEquals(0, Collections.binarySearch(demoList, 1));

        //public static int indexOfSubList(List<?

> source, List<?> target)
        // public static int lastIndexOfSubList(List<?> source, List<?> target)
    }

置换

    @Test
    public void testSwap() {
        List<Integer> demoList = new ArrayList<Integer>(Arrays.asList(3, 2, 1));

        //public static void swap(List<?

> list, int i, int j)
        Collections.swap(demoList, 0, 2);
        assertEquals(1, demoList.get(0).intValue());
        //public static void rotate(List<?> list, int distance)  //指定距离轮换
        //public static <T> boolean replaceAll(List<T> list, T oldVal, T newVal)  //指定替换
    }

拷贝

    @Test
    public void testCopy() {
        List<Integer> demoList = new ArrayList<Integer>(Arrays.asList(3, 2, 1));

        //public static <T> void copy(List<? super T> dest, List<? extends T> src)
        //注意目的List的size最少要等于src的size
        List<Integer> copyList = new ArrayList<Integer>(Arrays.asList(1, 2, 3));
        Collections.copy(copyList, demoList);
        assertEquals(3, copyList.size());
        assertEquals(3, copyList.get(0).intValue());

        copyList.add(4);
        assertEquals(3, demoList.size());
    }

比較

    @Test
    public void testCompary() {
        List<Integer> demoList = new ArrayList<Integer>(Arrays.asList(3, 2, 1));
        assertEquals(1, Collections.min(demoList).intValue());
        assertEquals(3, Collections.max(demoList).intValue());

        List<Integer> compareList = new ArrayList<Integer>(Arrays.asList(5, 6, 7));
        //Returns true if the two specified collections have no elements in common
        assertTrue(Collections.disjoint(demoList, compareList));
    }

创造不同的集合

    @SuppressWarnings("unused")
    @Test
    public void testCreate() {
        List<Integer> demoList = new ArrayList<Integer>(Arrays.asList(3, 2, 1));

        //空对象 size=0  无加入方法
        List<Integer> emptyList = Collections.emptyList();

        //返回一个仅仅包括指定对象的不可变列表。
        List<Integer> singletonList = Collections.singletonList(1);

        //返回指定列表的一个动态类型安全视图。
        List<Integer> checkedList = Collections.checkedList(demoList, Integer.class);

        //返回指定列表的不可改动视图。
        List<Integer> unmodifiableList = Collections.unmodifiableList(demoList);

        //返回指定列表支持的同步（线程安全的）列表。
        List<Integer> synchronizedList = Collections.synchronizedList(demoList);

        synchronized (synchronizedList) {
            Iterator<Integer> i = synchronizedList.iterator(); // Must be in synchronized block
            while (i.hasNext())
                i.next();
        }

        //map set sortedMap sortedSet
    }

环境 jdk1.6 window7 junit4