要点Java20 java.util.Collections

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java.util.Collections 集合帮助类

示例程序(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());    }

比较

    @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  无添加方法        List<Integer> emptyList = Collections.emptyList();        //返回一个只包含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修改视图。        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

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