关于集合的筛选，利用CollectionUtils筛选出符合条件的Collection集合

在工作中，很多时候要使用到集合筛选，而自己又不愿意写循环，在网上找到一种方法 CollectionUtils.select，

下面是个人的理解：

    /**
     * Selects all elements from input collection which match the given
     * predicate into an output collection.
     * 

     * A null predicate matches no elements.
     *
     * @param   the type of object the {@link Iterable} contains
     * @param inputCollection  the collection to get the input from, may not be null
     * @param predicate  the predicate to use, may be null
     * @return the elements matching the predicate (new list)
     * @throws NullPointerException if the input collection is null
     */
    public static  Collection select(final Iterable inputCollection,
                                           final Predicate predicate) {
        final Collection answer = inputCollection instanceof Collection ?
                new ArrayList(((Collection) inputCollection).size()) : new ArrayList();
        return select(inputCollection, predicate, answer);
    }

先查看这个方法，需要两个参数，Collection inputCollection为输出集合,如果对泛型边界有兴趣，可以查看effective java 第28条 利用有限制通配符来提升API的灵活性，Predicate predicate 是什么意思呢？

public interface Predicate {

    /**
     * Use the specified parameter to perform a test that returns true or false.
     *
     * @param object  the object to evaluate, should not be changed
     * @return true or false
     * @throws ClassCastException (runtime) if the input is the wrong class
     * @throws IllegalArgumentException (runtime) if the input is invalid
     * @throws FunctorException (runtime) if the predicate encounters a problem
     */
    public boolean evaluate(T object);

}

这是一个接口，只有一个evaluate方法，从字面意思是比较的意思，返回 true or false ,如何比较，由子类实现

可以写一个PO类：

public class Student {
	private Integer number;
	private String name;
	private Integer score;
	
	public Student() {
		super();
	}
	public Student(Integer number, String name, Integer score) {
		super();
		this.number = number;
		this.name = name;
		this.score = score;
	}
	public Integer getNumber() {
		return number;
	}
	public void setNumber(Integer number) {
		this.number = number;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public Integer getScore() {
		return score;
	}
	public void setScore(Integer score) {
		this.score = score;
	}
	@Override
	public String toString() {
		return "Student [number=" + number + ", name=" + name + ", score="
				+ score + "]";
	}
}

在写两个比较实现：

/**
 * 如果超过设定的integer 就返回true
 * 
 * @author Administrator
 *
 */
public class MoreThanPredicate implements Predicate{

	private final Integer score;
	
	public MoreThanPredicate(Integer score){
		this.score = score;
	}
	
	
	@Override
	public boolean evaluate(Student object) {
		
		return object.getScore() > score;
	}

}

/**
 * 相同比较器
 * @author Administrator
 *
 */
public class EqualPredicate implements Predicate{
	
	//学号相同就为相同一个学生
	private final Integer number;
	
	//初始化必须赋值
	EqualPredicate(Integer number){
		this.number = number;
	}
	
	@Override
	public boolean evaluate(Student object) {
		//由于是对象类型，所以不用==号的方式
		return object.getNumber().equals(number);
	}
	

}

下面是测试：

	/**
	 * 查找分数大于60的学生
	 * @param args
	 */
	public static void main(String[] args) {
		Student[]  ss = {new Student(0, "123", 99),new Student(1, "789", 59),new Student(3, "110", 80)};
		Predicate p = new MoreThanPredicate(60);
		Collection c = CollectionUtils.select(Arrays.asList(ss), p);
		System.out.println(c);
	}

[Student [number=0, name=123, score=99], Student [number=3, name=110, score=80]]

	/**
	 * 查找学号为1的同学
	 * @param args
	 */
	public static void main(String[] args) {
		Student[]  ss = {new Student(0, "123", 99),new Student(1, "789", 59),new Student(3, "110", 80)};
		Predicate p = new EqualPredicate(1);
		Collection c = CollectionUtils.select(Arrays.asList(ss), p);
		System.out.println(c);
	}

[Student [number=1, name=789, score=59]]

可以看出，很简洁：

在这个基础之上还可以继续扩展:
 

public class BeanEqualPredicate implements Predicate {

	private final String field;
	private final Object param;
	
	BeanEqualPredicate(String field,Object param){
		this.field = field;
		this.param = param;
	}
	
	
	@Override
	public boolean evaluate(T object) {
		
		try {
			Object propertyValue = PropertyUtils.getProperty(object, field);
			return propertyValue == param || propertyValue.equals(param);
		} catch (IllegalAccessException | InvocationTargetException
				| NoSuchMethodException e) {
			throw new RuntimeException(e);
		}
	}
}

	/**
	 * 动态查找
	 * @param args
	 */
	public static void main(String[] args) {
		Student[]  ss = {new Student(0, "123", 99),new Student(1, "789", 59),new Student(3, "110", 80)};
		System.out.println(select(Arrays.asList(ss),"number",1));
	}
	
	private static  Collection select(Collection c,String field,Object param){
		BeanEqualPredicate predicate = new BeanEqualPredicate<>(field, param);
		return CollectionUtils.select(c, predicate);
	}
	
}

[Student [number=1, name=789, score=59]]

经过这样封装，具有更好的灵活性，当然，如果需要，你可以继续扩展.