java基础类型用法

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ArrayList

    public void lists() { 
            List list = new ArrayList();
            list.add("demo1");
            list.add("demo2");
            list.add("demo1");
            list.add("demo2");
            int i = list.indexOf("demo2");
            System.out.println("list集合中所有的元素: " + list);
            list.forEach((e) -> {
                System.out.print(e + ", ");
            });
            System.out.println("************************");
            
            //toArray() 将 arraylist 转换为数组。
            Object[] obj = list.toArray();
            for(Object item : obj) {
                System.out.print(item+", ");
            }
            System.out.println("************************");

            System.out.println("第一-次出现demo2的索引位置是: " + i);
            i = list.indexOf("demo3" );
            System.out.println( "第一-次出现demo3的索引位置是: " + i);

            //下标获取值
            System.out.println("index1:"+list.get(1));

            //删除指定索引的对象
            list.remove(2);
            System.out.println("删除后的list: " + list);

            //在指定位置插入元素，后面的元素都往后移一个元素。
            list.add(1,"demo5");
            System.out.println("插入元素后的list: " + list);

            //在索引为index位置的元素替换为element元素
            list.set(2,"demo3");
            System.out.println("插入元素后的list: " + list);

            //返回从索引fromIndex到toIndex的元素集合，包左不包右
            List list2 = new ArrayList();
            list2=list.subList(1,3);
            System.out.println("list22222: " + list2);  
    }

List循环下标的2种方式

List<String> list = new ArrayList<>();
list.add("1");
list.add("2");
list.add("3");
list.add("4");
list.add("5");
 
Stream.iterate(0, i -> i + 1).limit(list.size()).forEach(i -> {
    System.out.println(list.get(i));
}); 
 
IntStream.range(0,list.size()).forEach(i->{
    System.out.println(list.get(i));
});

Map HashMap

    public void maps(){
        HashMap<Integer, String> Sites = new HashMap<Integer, String>();
        // 添加键值对
        Sites.put(1, "Google");
        Sites.put(2, "Runoob");
        Sites.put(3, "Taobao");
        Sites.put(4, "Zhihu");
        System.out.println(Sites);
        System.out.println(Sites.get(2));
        Sites.remove(3);
        System.out.println(Sites);

        // 输出 key 和 value
        for (Integer i : Sites.keySet()) {
            System.out.println("key: " + i + " value: " + Sites.get(i));
        }
    }

public void a1(){
        Map<String,Object> map = new HashMap<>();
        map.put("a","尺寸");
        map.put("b","7英寸");
        map.put("c",2);
        map.put("d",5);
        System.out.println(map);//{a=尺寸, b=7英寸, c=2, d=5}
        System.out.println(map.get("b"));//7英寸
        System.out.println(map.get(1)); //null
        map.forEach((key, value) -> {
            System.out.print(key + "=" + value + " ");
        });
    }

迭代器iterator

    public void Iterator(){
        // 创建集合
        ArrayList<String> sites = new ArrayList<String>();
        sites.add("Google");
        sites.add("Runoob");
        sites.add("Taobao");
        sites.add("Zhihu");
        sites.add("aaa");
        sites.add("bbbbbb");

        // 获取迭代器
        Iterator<String> it = sites.iterator();

        // 输出集合中的第一个元素
        System.out.println(it.next());
        System.out.println(it.next());
        while(it.hasNext()) {
            String i = it.next();
            if(i.length() < 4) {
                it.remove();  // 删除小于 10 的元素
            }
        }
        //[Google, Runoob, Taobao, Zhihu, bbbbbb]
        System.out.println(sites);
    }

泛型

    @Test
    public void tclass(){
        Integer[] intArray = { 1, 2, 3, 4, 5 };
        Double[] doubleArray = { 1.1, 2.2, 3.3, 4.4 };
        String[] strArray = { "Haa", "Ebb", "L", "L", "O" };

        System.out.print("int:");
        for(int x : intArray ){
            System.out.print( x + ",");
        }
        System.out.print("double:");
        for(double x : doubleArray){
            System.out.print( x + ",");
        }
        System.out.print("string:");
        for(String x : strArray){
            System.out.print( x + ",");
        }

        List<String> name = new ArrayList<String>();
        List<Integer> age = new ArrayList<Integer>();
        List<Number> number = new ArrayList<Number>();

        name.add("icon");
        age.add(18);
        number.add(314);

        //getUperNumber(name);//1
        getUperNumber(age);//2
        getUperNumber(number);//3
    }

    //Integer继承与Number
    public static void getUperNumber(List<? extends Number> data) {
        System.out.println("data :" + data.get(0));
    }

}

多维map

    @Test
    public void a2(){
        Map<String,Object> map1 = new HashMap<>();
        map1.put("a","尺寸");
        map1.put("b","7英寸");
        map1.put("c",2);
        map1.put("d",5);

        Map<String,Object> map2 = new HashMap<>();
        map2.put("a","1尺寸");
        map2.put("b","3英寸");
        map2.put("c",6);
        map2.put("d",7);

        System.out.println(map1);
        System.out.println(map2);
        System.out.println("map2:" + map2.get("b"));
        
        //Map<String,Object>> map = new HashMap<>();
        //如果如上这样定义，下面是不会有foreach和get等用法的
        Map<String,Map<String,Object>> map = new HashMap<>();
        map.put("0",map1);
        map.put("1",map2);
        System.out.println(map);

        map.forEach((k,v)->{
            System.out.print(k + "=" + v + " ");
            System.out.println("v:" + v.get("b"));
        });
    }

    @Test
    public void aa() {
        String[] runoobs = new String[3];
        runoobs[0] = "菜鸟教程";
        runoobs[1] = "菜鸟工具";
        runoobs[2] = "菜鸟笔记";
        //runoobs[3] = "菜鸟笔记";//会报错
        for (String str:runoobs){
            System.out.println(str+"\n");
        }
        Map<String,Object>[] map = new HashMap[3];
        map[0] = new HashMap<String,Object>();
        map[0].put("a","尺寸");
        map[0].put("b","7英寸");
        map[0].put("c",2);


        map[1] = new HashMap<String,Object>();
        map[1].put("a","5尺寸");
        map[1].put("b","12英寸");
        map[1].put("c",7);
        for (Map<String,Object> obj:map){
            obj.forEach((k,v)->{
                System.out.print(k + "=" + v + " ");//能打印出来，但也会报错
                //System.out.println("v:" + v.get("b"));    //不存在get
            });
        }  
    }