描述学生，map容器，学生为键，地址为值，获取map中的元素。
public class MapDemo { public static void main(String[] args) { HashMap<Student, String> hm = new HashMap<Student, String>(); hm.put(new Student("lisi1", 1), "北京"); hm.put(new Student("lisi2", 2), "上海"); hm.put(new Student("lisi3", 3), "天津"); hm.put(new Student("lisi4", 4), "武汉"); //第一种取出方式 keySet Set<Student> keySet = hm.keySet(); Iterator<Student> it = keySet.iterator(); while (it.hasNext()) { Student stu = it.next(); String addr = hm.get(stu); System.out.println(stu + "--" + addr); } //第二种取出方式 entrySet Set<Map.Entry<Student, String>> entrySet = hm.entrySet(); Iterator<Map.Entry<Student, String>> i = entrySet.iterator(); while (i.hasNext()) { Map.Entry<Student, String> stu = i.next(); System.out.println(stu.getKey() + "--" + stu.getValue()); } }}class Student implements Comparable<Student> { private String name; private int age; public Student(String name, int age) { this.name = name; this.age = age; } @Override public boolean equals(Object obj) { if (!(obj instanceof Student)) throw new ClassCastException("类型不匹配"); Student s = (Student) obj; return this.name.equals(s.name) && this.age == s.age; } @Override public int hashCode() { return name.hashCode() + age * 34; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return "Student{" + "name='" + name + '\'' + ", age=" + age + '}'; } @Override public int compareTo(Student o) { int num = new Integer(this.age).compareTo(new Integer(o.age)); if (num == 0) { return this.name.compareTo(o.name); } return num; }}
排序年龄：
public class MapDemo { public static void main(String[] args) { TreeMap<Student, String> tm = new TreeMap<Student, String>(); tm.put(new Student("lisi4", 1), "武汉"); tm.put(new Student("lisi1", 4), "北京"); tm.put(new Student("lisi3", 2), "天津"); tm.put(new Student("lisi2", 3), "上海"); Set<Map.Entry<Student, String>> entrySet = tm.entrySet(); Iterator<Map.Entry<Student, String>> it = entrySet.iterator(); while (it.hasNext()) { Map.Entry<Student, String> stu = it.next(); System.out.println(stu.getKey() + "--" + stu.getValue()); } }}class Student implements Comparable<Student> { private String name; private int age; public Student(String name, int age) { this.name = name; this.age = age; } @Override public boolean equals(Object obj) { if (!(obj instanceof Student)) throw new ClassCastException("类型不匹配"); Student s = (Student) obj; return this.name.equals(s.name) && this.age == s.age; } @Override public int hashCode() { return name.hashCode() + age * 34; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return "Student{" + "name='" + name + '\'' + ", age=" + age + '}'; } @Override public int compareTo(Student o) { int num = new Integer(this.age).compareTo(new Integer(o.age)); if (num == 0) { return this.name.compareTo(o.name); } return num; }}
姓名排序：
public class MapDemo { public static void main(String[] args) { TreeMap<Student, String> tm = new TreeMap<Student, String>(new stuComparator()); tm.put(new Student("lisi4", 1), "武汉"); tm.put(new Student("lisi1", 4), "北京"); tm.put(new Student("lisi3", 2), "天津"); tm.put(new Student("lisi2", 3), "上海"); Set<Map.Entry<Student, String>> entrySet = tm.entrySet(); Iterator<Map.Entry<Student, String>> it = entrySet.iterator(); while (it.hasNext()) { Map.Entry<Student, String> stu = it.next(); System.out.println(stu.getKey() + "--" + stu.getValue()); } }}class Student implements Comparable<Student> { private String name; private int age; public Student(String name, int age) { this.name = name; this.age = age; } @Override public boolean equals(Object obj) { if (!(obj instanceof Student)) throw new ClassCastException("类型不匹配"); Student s = (Student) obj; return this.name.equals(s.name) && this.age == s.age; } @Override public int hashCode() { return name.hashCode() + age * 34; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return "Student{" + "name='" + name + '\'' + ", age=" + age + '}'; } @Override public int compareTo(Student o) { int num = new Integer(this.age).compareTo(new Integer(o.age)); if (num == 0) { return this.name.compareTo(o.name); } return num; }}class stuComparator implements Comparator<Student> { @Override public int compare(Student o1, Student o2) { int num = o1.getName().compareTo(o2.getName()); if (num == 0) return new Integer(o1.getAge()).compareTo(new Integer(o2.getAge())); return num; }}
获取字符串("aabbbcccccdfff")中字母出现的次数：
希望打印结果：a(1)c(2)......
有映射关系先想到map集合。
1.将字符串转换成字符数组，因为要对每一个字母进行操作
2.定义map集合，打印结果有顺序，使用treemap
3.遍历字符数组，将每一个字母作为键去查map集合，返回null，就将该字母和1存入map集合中，返回不是null，获取次数加1，在存入集合，覆盖原来的键值对。
4.将map集合的数据变成指定的字符串形式返回。
public class Demo { public static void main(String[] args) { String s = charCount("aa+++___bbbcccccdfff"); System.out.println(s); } public static String charCount(String str) { char[] chs = str.toCharArray(); TreeMap<Character, Integer> tm = new TreeMap<Character, Integer>(); int count = 0; for (int i = 0; i < chs.length; i++) { if (!(chs[i]>='a' && chs[i]<='z' || chs[i]>='A' && chs[i]<='Z')) continue; Integer value = tm.get(chs[i]); if (value != null) count = value; count++; tm.put(chs[i], count); count = 0; } StringBuilder sb = new StringBuilder(); Set<Map.Entry<Character, Integer>> entrySet = tm.entrySet(); Iterator<Map.Entry<Character, Integer>> it = entrySet.iterator(); while (it.hasNext()) { Map.Entry<Character, Integer> me = it.next(); Character ch = me.getKey(); Integer value = me.getValue(); sb.append(ch + "(" + value + ")"); } return sb.toString(); }}
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