sort

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List<String> list = Arrays.asList("0002","0004","0001","0006","0003","0007","0009"); 
list.sort(new Comparator<String>() {
    @Override
    public int compare(String o1, String o2) {
      return Integer.parseInt(o1) > Integer.parseInt(o2)?1:-1; //升序
      //return Integer.parseInt(o1) < Integer.parseInt(o2)?1:-1; //降序
    }
  });

Collections.sort

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Collections.sort(list, new Comparator<String>() {
  @Override
  public int compare(String o1, String o2) {
    return Integer.parseInt(o1) > Integer.parseInt(o2)?1:-1; //升序
    //return Integer.parseInt(o1) < Integer.parseInt(o2)?1:-1; //降序
  }
});

Stream

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List<String> list = Arrays.asList("0002","0004","0001","0006","0003","0007","0009");
list = list.stream().sorted(new Comparator<String>() {
  @Override
  public int compare(String o1, String o2) {
    return Integer.parseInt(o1) > Integer.parseInt(o2)?1:-1; //升序
    //return Integer.parseInt(o1) < Integer.parseInt(o2)?1:-1; //降序
  }
}).collect(Collectors.toList());
for (String s : list) {
  System.out.println(s);
}

Comparable

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class Person implements Comparable<Person> {
    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
  
    @Override
    public int compareTo(Person p) {
        return this.age > p.age ? 1:-1; //升序
        // return this.age > p.age ? -1:1; //降序
    }
}

List<Person> personList = new ArrayList<Person>(){{
  add(new Person("li", 19));
  add(new Person("wang", 29));
  add(new Person("meng", 12));
}};

//排序
Collections.sort(personList);

for (Person person : personList) {
  System.out.println(person.toString());
}