Lambda表达式操做集合

时间  2020-01-25 标签 lambda 表达式 操 做 集合

Lambda表达式操做集合

本博客数据来源：示例数据java

示例１：对List进行迭代

public static void main(String[] args) {
    List<Dept> deptList = Arrays.asList(new Dept(1, "OPERATION", "BOSTON"),
            new Dept(3, "SALES", "CHICAGO"), new Dept(4, "ACCOUNTING", "NEW YORK"),
	new Dept(2, "RESEARCH", "DALLAS"));
deptList.forEach(item -> System.out.println(item));
deptList.forEach(dept-> dept.getEmps().forEach(System.out::println)); //遍历双重循环
}

示例2：对List进行排序

public class LambdaTest{
    List<Dept> deptList;
    @Before
    public void before() {
        deptList = Arrays.asList(new Dept(1, "OPERATION", "BOSTON"),
 		new Dept(2, "RESEARCH", "TOKYO"), new Dept(3, "SALES", "CHICAGO"),
             new Dept(4, "ACCOUNTING", "NEW YORK"), new Dept(2, "RESEARCH", "DALLAS"));
    }
    @Test
    public void fun1() {  //基本排序
        deptList.sort((dept1, dept2) -> dept1.getDname().compareTo(dept2.getDname()));
        deptList.forEach(item -> System.out.println(item));
    }
    @Test
    public void fun2() { //排序完逆序
        Comparator<Dept> comparator = (dept1, dept2) -> 
			dept1.getDname().compareTo(dept2.getDname());
        deptList.sort(comparator.reversed());
        deptList.forEach(item -> System.out.println(item));
    }
    public static int compareByDnameThenLoc(Dept dept1, Dept dept2) {
        if (dept1.getDname().equals(dept2.getDname())) {
            return dept1.getLoc().compareTo(dept2.getLoc());
        } else {
            return dept1.getDname().compareTo(dept2.getDname());
        }
    }
    @Test
    public void fun3() {//使用静态方法的引用来排序
        deptList.sort(LambdaTest::compareByDnameThenLoc);
        deptList.forEach(item -> System.out.println(item));
    }
    @Test
    public void fun4() {//多条件排序
        deptList.sort((dept1, dept2) -> {
            if (dept1.getDname().equals(dept2.getDname())) {
                return dept1.getLoc().compareTo(dept2.getLoc());
            } else {
                return dept1.getDname().compareTo(dept2.getDname());
            }
        });
        deptList.forEach(item -> System.out.println(item));
    }
    @Test
    public void fun5() {//提取Comparator排序
        Collections.sort(deptList, Comparator.comparing(Dept::getDname));
        deptList.forEach(item -> System.out.println(item));
    }
    @Test
    public void fun6() {//多条件组合排序
        Collections.sort(deptList, Comparator.comparing(Dept::getDname)
				.thenComparing(Dept::getLoc));
        deptList.forEach(item -> System.out.println(item));
    }
}

示例3：List元素去除重复

public static void main(String[] args) throws IOException {
	List<String> list = Arrays.asList("aa","bb","cc","aa","dd");
list.stream().distinct().collect(Collectors.toList())
			.forEach(System.out::println);
}

示例4：分别对List的每个元素进行处理

public static void main(String[] args) {
    List<String> list = Arrays.asList("zhangsan", "lisi", "wanger", "mzai");
    list.stream().map(item -> item+" : "+item.toUpperCase()).forEach(System.out::println);
}

示例5：对List中的元素进行转换

public static void main(String[] args) throws IOException {
	Dept accounting = new Dept((byte)10, "ACCOUNTING","NewYork");
	Dept research = new Dept((byte)20, "RESEARCH","Dallas");
	Dept sales = new Dept((byte)30, "SALES","Chicago");
	Dept operations = new Dept((byte)40, "OPERATIONS","Boston");
	List<Dept> list = new ArrayList<>();
	list.add(accounting);
	list.add(research);
	list.add(sales);
	list.add(operations);
	List<String> dnameList = list.stream().map(item->item.getDname())
								.collect(Collectors.toList());
dnameList.forEach(System.out::println);
}

示例6：对List中的元素进行过滤

public static void main(String[] args) {
    List<String> list = Arrays.asList("zhangsan", "lisi", "wanger", "mzai");
    List<String> res = list.stream().filter(item -> item.length() > 5)
			.collect(Collectors.toList());
res.forEach(System.out::println);

List<Integer> numberList = Arrays.asList(1, 2, 3, 4, 5);
numberList.stream().filter(i -> i % 2 == 0).forEach(System.out::println);
}

示例7：将List中的字符串换成小写并用#号连接起来

  • 实现一web

    public static void main(String[] args) {
    List<String> list = Arrays.asList("USA", "FRANCE", "GERMANY", "ITALY", "CANADA");
    String res = list.stream().map(x -> x.toLowerCase()).collect(Collectors.joining("#"));
    System.out.println(res);
}

  • 实现二：数组

    public class LambdaTest {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
        filter1(numbers, x -> (int) x % 2 == 0);  // 2 4
List<String> words = Arrays.asList("a", "ab", "abc", "abcd", "abcde");
        filter2(words, x -> ((String) x).length() > 2);  //abc abcd abcde
    }
    public static void filter1(List list, Predicate condition) {
        list.forEach(x -> {
            if (condition.test(x)) {
                System.out.print(x + " ");
            }
        });
    }
    public static void filter2(List list, Predicate condition) { //更佳的方式
        list.stream().filter(x -> condition.test(x)).forEach(x -> {
            System.out.print(x + " ");
        });
    }
}

  • 实现三：app

    public static void main(String[] args) throws IOException {
	Dept accounting = new Dept((byte)10, "ACCOUNTING","NewYork");
	Dept research = new Dept((byte)10, "RESEARCH","Dallas");
	Dept sales = new Dept((byte)10, "SALES","Chicago");
	Dept operations = new Dept((byte)10, "OPERATIONS","Boston");
	List<Dept> list = new ArrayList<>();
	list.add(accounting);
	list.add(research);
	list.add(sales);
	list.add(operations);
	
	List<Emp> empList = list.stream().filter(item->item.getDname().contains("E"))
		.map(item->{
			Emp emp = new Emp();
			emp.setDeptno(item.getDeptno());
			return emp;
		}).collect(Collectors.toList());	
	empList.forEach(System.out::println); 
}

  • 实现四：ide

    public static void main(String[] args) throws IOException {
	Dept accounting = new Dept((byte)10, "ACCOUNTING","NewYork");
	Dept research = new Dept((byte)20, "RESEARCH","Dallas");
	Dept sales = new Dept((byte)30, "SALES","Chicago");
	Dept operations = new Dept((byte)40, "OPERATIONS","Boston");
	
	Map<Integer, Dept> map = new HashMap<>();
	map.put(10, accounting);
	map.put(20, research);
	map.put(30, sales);
	map.put(40, operations);
	
	List<Dept> empList = map.entrySet().stream().map(item->{
		Dept temp = item.getValue();
		temp.setDname(temp.getDname().toLowerCase());
		return temp;
	}).collect(Collectors.toList());
	empList.forEach(System.out::println);
}

示例8：对List中的元素进行聚合/规约：reduce

public static void main(String[] args) {
    List<Integer> list = Arrays.asList(10,20,30,40,50);
    Integer res = list.stream().reduce((sum, item) -> sum + item).get();
    System.out.println(res);
}

示例9：

public static void main(String[] args) {
        List<Integer> list = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
        Stream<Integer> stream = list.stream();
// stream.reduce(new BinaryOperator<Integer>() {
// @Override
// public Integer apply(Integer a, Integer b) {
// return a+b;
// }
// });
        Optional<Integer> optional = stream.reduce((a, b) -> a + b);
        Integer integer = optional.get();
        System.out.println(integer);
    }

示例１0：求List的最大值、最小值、平均值

public static void main(String[] args) {
    List<Integer> list = Arrays.asList(3, 5, 29, 7,17, 2, 19);
    IntSummaryStatistics stats = list.stream().mapToInt((x) -> x).summaryStatistics();
    System.out.println("最大值：" + stats.getMax());
    System.out.println("最小值：" + stats.getMin());
    System.out.println("和：" + stats.getSum());
    System.out.println("平均值：" + stats.getAverage());
}

示例11：遍历Map

public static void main(String[] args) {
    Map<String,Integer> map =new HashMap<>();
    map.put("zhangsan",17);
    map.put("lisi",18);
    map.put("wanger",16);
    map.put("mazi",19);
    map.forEach((k,v)->System.out.println(k+" : "+v));
}

示例12：防止集合中的元素为null。filter(Objects::nonNull)

public static void main(String[] args) throws IOException {
	Dept accounting = new Dept((byte) 10, "ACCOUNTING", "NewYork");
	Dept research = new Dept((byte) 20, "RESEARCH", "Dallas");
	Dept sales = new Dept((byte) 30, "SALES", "Chicago");
	Dept operations = new Dept((byte) 40, "OPERATIONS", "Boston");

	List<Dept> list = new ArrayList<>();
	list.add(accounting);
	list.add(research);
	list.add(sales);
	list.add(null);
	list.add(operations);

	Optional.ofNullable(list).orElse(Collections.emptyList())  //集合null过滤
		.stream().filter(Objects::nonNull).forEach(item->{ //集合元素null过滤
			System.out.println(item.getDname());
	});
}

示例13：

public static void main(String[] args) throws IOException {
	Dept accounting = new Dept((byte) 10, "ACCOUNTING", "NewYork");
	Dept research = new Dept((byte) 20, "RESEARCH", "Dallas");
	Dept sales = new Dept((byte) 30, "SALES", "Chicago");
	Dept operations = new Dept((byte) 40, "OPERATIONS", "Boston");

	Map<Integer, Dept> map = new HashMap<>();
	map.put(10, accounting);
	map.put(20, research);
	map.put(30, sales);
	map.put(null, operations);
	
map.keySet().stream().filter(Objects::nonNull)
			.forEach(System.out::println);
}

示例14：转换数组为List

  • 使用java.util.Arrays工具类中的asList()方法（这个不是Java8中新增的内容）:svg

    String[] arrays = new String[]{"a", "b", "c"};
List<String> listStrings = Arrays.asList(arrays);

  • 使用Stream中的Collector收集器，代码:工具

    String[] arrays = new String[]{"a", "b", "c"};
List<String> listStringOne = Stream.of(arrays).collector(Collectors.toList());
List<String> listStringTwo = Arrays.stream(arrays).collector(Collectors.toList());

示例15：转换List为数组

  • 使用Stream:spa

    String[] ss = listStrings.stream().toArray(String[]::new);

  • 使用List中的toArray()方法.net

    String[] sss = listStrings.toArray(new String[listStrings.size()]);

示例16：将集合改为数组

public static void main(String[] args) {
    String[] array = List.of("aa", "bb").toArray(String[]::new);
    for (String item : array) 
        System.out.println(item);
}
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