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关键字: List.sort(), collect(), filter()

问题发现

[Java 1.8]

在测试代码时发现使用

ArrayList.stream().filter(...).collect(Collectors.toList()).sort()

对List进行过滤排序时，并没有起到想要的效果

寻找原因

1. collect

collect 会返回处理后的List，并不会在原有基础上做修改，在上文中使用时并没有接收处理后的List，所以看起来没做任何处理。

先修正为

aNewList = ArrayList.stream().filter(...).collect(Collectors.toList()).sort()

,看结果还是失败了！那应该还有其他原因。

2. 先拆分试试看

aNewList = ArrayList.stream().filter(...).collect(Collectors.toList())
aNewList.sort()

过滤排序成功，查看List.sort函数，发现sort并没有返回值！找到原因了：

写成

filter().collect().sort()

时，是在collect()的返回list上进行排序，但是sort并不会有返回值。相当于在一个临时的List上做了过滤排序，但结束后该List就被销毁了，你也无法获取其结果，相当做了无用功。

3. 推荐写法

// 拆分写
aNewList = ArrayList.stream().filter(...).collect(Collectors.toList())
aNewList.sort()
// 用sorted函数
aNewList = ArrayList.stream().filter(...).sorted().collect(Collectors.toList())

测试代码

// point 类
public class point {

	private String pointCode;
	
	private String pointSize;

	point(String pointcode, String pointsize) {
		this.pointCode = pointcode;
		this.pointSize = pointsize;
	}

	public String getPointCode() {
		return pointCode;
	}

	public void setPointCode(String pointCode) {
		this.pointCode = pointCode;
	}

	public String getPointSize() {
		return pointSize;
	}

	public void setPointSize(String pointSize) {
		this.pointSize = pointSize;
	}
}

// 过滤操作
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;
import Study1009.point;

public class testfilter {
	public static void main(String []args) {
		List<String> filtereList = Arrays.asList("S", "R", "G", "T");
		point defect1 = new point("a", "G");
		point defect2 = new point("b", "R");
		point defect3 = new point("c", "T");
		point defect4 = new point("d", "S");
		point defect5 = new point("e", "I");
		
		List<point> defectList = new ArrayList<point>();
		defectList.add(defect1);
		defectList.add(defect2);
		defectList.add(defect3);
		defectList.add(defect4);
		defectList.add(defect5);
		
		// 过滤失败
		defectList.stream().filter(O -> filtereList.contains(O.getPointSize())).collect(Collectors.toList());
		
		// 过滤成功
		defectList = defectList.stream().filter(O -> filtereList.contains(O.getPointSize()))
				.collect(Collectors.toList());
		
		// 过滤排序失败 error: Type mismatch: cannot convert from void to List<Defect>
//		defectList = defectList.stream().filter(O -> filtereList.contains(O.getPointSize()))
//				.collect(Collectors.toList()).sort(Comparator.comparingInt(O -> judgeList.indexOf(O.getPointSize())));
		
//		 过滤成功
		defectList = defectList.stream().filter(O -> filtereList.contains(O.getPointSize()))
				.collect(Collectors.toList());
		defectList.sort(Comparator.comparingInt(O -> filtereList.indexOf(O.getPointSize())));
		
		
		// 过滤成功， 但要接收collect的返回结果，否则原List没有发生改变
		defectList = defectList.stream().filter(O -> filtereList.contains(O.getPointSize()))
				.sorted(Comparator.comparingInt(O -> filtereList.indexOf(O.getPointSize()))).collect(Collectors.toList());
		
		
		for (int i=0; i<defectList.size(); i++) {
			System.out.println(defectList.get(i).getPointCode() + "-" + defectList.get(i).getPointSize() + " ");
		}

	}
}