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3-24集合

鱼板番茄 2022-03-24 阅读 47
java

1、有如下map: Map<String,String> map = new HashMap<>(); map.put(“aaa”,“111”); map.put(“bbb”,“111”); map.put(“ccc”,“111”); map.put(“ddd”,“222”);

使用3

 Map<String,String> map = new HashMap<>();
        map.put("aaa","111");
        map.put("bbb","111");
        map.put("ccc","111");
        map.put("ddd","111");

        Set<String> keys = map.keySet();
        Iterator<String> itKey = keys.iterator();
        while (itKey.hasNext()){
            String key = itKey.next();
            String value = map.get(key);
            System.out.println(key + ":" + value);
        }

        Collection<String> values = map.values();
        Iterator<String> valueIt = values.iterator();
        while (valueIt.hasNext()){
            String value = valueIt.next();
            System.out.println(value);
        }

        Set<Map.Entry<String, String>> entries = map.entrySet();
        Iterator<Map.Entry<String, String>> entyrIt = entries.iterator();
        while (entyrIt.hasNext()){
            Map.Entry<String,String> entry = entyrIt.next();
            String key = entry.getKey();
            String value = entry.getValue();
            System.out.println(entry);
        }

种方式,遍历并打印

2、定义集合List, 依次将1,2,3,4,5,6,7,8,9 添加到集合中 使用Collections中的方法: 打乱顺序 求最大值 求最小值 升序排列 交换位置 将该集合所有元素拷贝到另一个集合中并打印

List<Integer> list = new ArrayList<>();
        for (int i = 0; i < 9; i++) {
            list.add(i+1);
        }
        System.out.println(list);
        Collections.shuffle(list);
        System.out.println(list);
        int max = Collections.max(list);
        System.out.println(max);
        int min = Collections.min(list);
        System.out.println(min);
        Collections.sort(list);
        System.out.println(list);
        Collections.swap(list,0,1);
        List<Integer> newList = new ArrayList<>();
        newList.add(1);
        newList.add(1);
        newList.add(1);
        newList.add(1);
        newList.add(1);
        newList.add(1);
        newList.add(1);
        newList.add(1);
        newList.add(1);
        Collections.copy(newList,list);
        System.out.println(newList);

3、编写代码,模拟如下异常: ArithmeticException类 - 算术异常 ArrayIndexOutOfBoundsException类 - 数组下标越界异常 NullPointerException - 空指针异常 ClassCastException - 类型转换异常 NumberFormatException - 数字格式异常 OutOfMemoryError - 内存溢出错误

//ArithmeticException类 - 算术异常
        int i1 = num/0;
        System.out.println(i1);
//ArrayIndexOutOfBoundsException类 - 数组下标越界异常
        int[] arrs = new int[2];
        arrs[3]=12;
        System.out.println(arrs[3]);
//NullPointerException - 空指针异常
        int i = null;
//ClassCastException - 类型转换异常
class A{}
class B extends A{}
class C extends B{}
        A a = new B();
        C c = (C)a;
//NumberFormatException - 数字格式异常
        int a = Integer.parseInt("abc");
        System.out.println(a);
//OutOfMemoryError - 内存溢出错误
        String str = "a";
        for (int i = 0; i < 77777777; i++) {
            str = str + str;
        }
        System.out.println(str);
4、分析以下需求,并用代码实现:
    (1)统计每个单词出现的次数
    (2)有如下字符串
    "If you want to change your fate I think you must come to the school to learn java"(用空格间隔)
    (3)打印格式:
        to=3
        think=1
        you=2
    String str ="If you want to change your fate I think you must come to the school to learn java";
        String[] arr = str.split("\\s+");
        for (int i = 0; i < arr.length; i++) {
            System.out.println(arr[i]);
        }

        HashMap<String,Integer> map = new HashMap<>();
        for (int i = 0; i < arr.length; i++) {
            if (!map.containsKey(arr[i])){
                map.put(arr[i],1);
            }else if (map.containsKey(arr[i])){
                int i1 = map.get(arr[i]);
                map.put(arr[i],i1+1 );
            }
        }
        System.out.println(map);

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