Java 开发常用判断、去重...

冷不防 2021-10-03 03:04 327阅读 0赞

#### 1.List中去除null ####

list.removeAll(Collections.singleton(null));  
此类 `Collections` 映入的包

import org.apache.commons.collections.CollectionUtils;

#### 2.字符串查用判空 ####

StringUtils.isBlank()  
StringUtils.isEmpty()

相对比较isBlank比isEmpty强大一点  
在做判空的时候isBlank能判断字符串中的String aa = " ";  
而isEmpty是不行的

#### 3.StringUtils引入的包 ####

import org.apache.commons.lang3.StringUtils;

#### 4.集合判空 ####

CollectionUtils.isNotEmpty();  
引入的包

import org.apache.commons.collections.CollectionUtils;

#### 5.List集合去重 ####

List<String> list = new ArrayList<>();
    list.add("你好");
    list.add("baby");
    list.add("你好");
    Set<String> set = new HashSet<String>();
    set.addAll(list);
    list.clear();
    list.addAll(set);

#### 6.实体对象不实现hashcode和equals方法也能实现去重逻辑 ####

`User`实体对象

public class User {
    	private String name;
    	private String age;
    	private String sex;
    	private Integer id;
    
    	public String getName() {
    		return name;
    	}
    
    	public void setName(String name) {
    		this.name = name;
    	}
    
    	public String getAge() {
    		return age;
    	}
    
    	public void setAge(String age) {
    		this.age = age;
    	}
    
    	public String getSex() {
    		return sex;
    	}
    
    	public void setSex(String sex) {
    		this.sex = sex;
    	}
    
    	public Integer getId() {
    		return id;
    	}
    
    	public void setId(Integer id) {
    		this.id = id;
    	}
    
    	public User(String name, String age, String sex, Integer id) {
    		this.name = name;
    		this.age = age;
    		this.sex = sex;
    		this.id = id;
    	}
    
    	public User() {
    	}
    }

List<User> list = new ArrayList<>();
    list.add(new User("小白","12","女",12));
    list.add(new User("李白","12","男",12));
    list.add(new User("baby","12","女",12));
    list.add(new User("小白","12","女",12));
    list.add(new User("小白","12","女",12));
    
    Set<User> set = new TreeSet<>(new Comparator<User>() {
        @Override
        public int compare(User user, User t1) {
            // 数据对比
            return (user.getName().compareTo(t1.getName()));
        }
    });
    set.addAll(list);

打印结果

User{name='baby', age='12', sex='女', id=12}
    User{name='小白', age='12', sex='女', id=12}
    User{name='李白', age='12', sex='男', id=12}

#### Set源码 ####

public interface Set<E> extends Collection<E> {
        int size();
    
        boolean isEmpty();
    
        boolean contains(Object var1);
    
        Iterator<E> iterator();
    
        Object[] toArray();
    
        <T> T[] toArray(T[] var1);
    
        boolean add(E var1);
    
        boolean remove(Object var1);
    
        boolean containsAll(Collection<?> var1);
    
        boolean addAll(Collection<? extends E> var1);
    
        boolean retainAll(Collection<?> var1);
    
        boolean removeAll(Collection<?> var1);
    
        void clear();
    
        boolean equals(Object var1);
    
        int hashCode();
    
        default Spliterator<E> spliterator() {
            return Spliterators.spliterator(this, 1);
        }
    }

这里用到`Comparator`这个类，`Comparator`比较器重写了`compareTo`的方法。`compareTo`这个方法只是一个对比，做不到去重的目的。因为Set的源码中实现了`hashcode`和`equals`方法，在加上`Comparator`就是可以实现去重了！