list.stream()基本应用-优快云博客

这篇博客介绍了Java中对字符串、List和自定义对象的各种操作，包括计算字符出现次数、移除List重复元素、筛选符合条件的对象、按属性去重以及将List转换为Map。同时，还展示了如何删除Map中value为null的键值。这些方法利用了Java 8的Stream API，提高了代码的简洁性和效率。

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参考 ianly梁炎 url:https://blog.youkuaiyun.com/ianly123/article/details/82658622

1、String中某个字符出现次数：

    public static int strCount(String str,String findByStr){
        String[] split = str.split("");
        return Arrays.asList(split).stream().filter(s -> s.equals(findByStr)).collect(Collectors.toList()).size();
    }

2、List<String>去除重复元素：

    public static List<String> removeRepEle(List<String> list){
        return list.stream().distinct().collect(Collectors.toList());
    }

eg:Object 对象

package com.ouyeel.platform.components.rzcore.foundation.service.csy.vo;
import java.io.Serializable;
/**Object对象*/
public class PaFlowInfo implements Serializable {
    private static final long serialVersionUID = 1L;
    private String serialNo;
    private String remark;
    public String getSerialNo() {
        return serialNo;
    }
    public void setSerialNo(String serialNo) {
        this.serialNo = serialNo;
    }
    public String getRemark() {
        return remark;
    }
    public void setRemark(String remark) {
        this.remark = remark;
    }
}

3、List<Object>筛选符合条件的对象

public static List<PaFlowInfo> getConditionList(List<PaFlowInfo>list){
           return list.stream().filter(paFlowInfo->paFlowInfo.getSerialNo().equals("serialNo_1")).collect(Collectors.toList());
       }

4、List<Object>根据对象属性(一个)，去除重复对象

public static List<PaFlowInfo> removeRepEleByOne(List<PaFlowInfo> list){
           return list.stream().collect(
                   Collectors.collectingAndThen(
                           Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(PaFlowInfo::getSerialNo))), ArrayList::new)
           );
       }

5、List<Object>根据对象属性(两个)，去除重复对象

public static List<PaFlowInfo> removeRepEleByTwo(List<PaFlowInfo> list){
        return list.stream().collect(
                Collectors. collectingAndThen(
                        Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(paFlowInfo -> paFlowInfo.getSerialNo() + ";" + paFlowInfo.getRemark()))), ArrayList::new)
        );
    }

6、List<Object>根据两个属性转换为Map集合(作为key的属性不可重复)

public static Map<String,String> transformMap(List<PaFlowInfo> list){
        return list.stream().collect(Collectors.toMap(paFlowInfo -> paFlowInfo.getSerialNo(), paFlowInfo -> paFlowInfo.getRemark()));
    }

7、从 List<Object> 中取出某个属性的组成 list 集合

public static List<String> assemblyNewList(List<PaFlowInfo>list){
        return list.stream().map(PaFlowInfo::getSerialNo).collect(Collectors.toList());
    }

8、从 List<Object> 中取出某个属性的组成 list 集合并去重

public static List<String> assemblyNewListRemoveRep(List<PaFlowInfo>list){
        return list.stream().map(PaFlowInfo::getSerialNo).distinct().collect(Collectors.toList());
    }

9、删除map中value 为null的键值

public static void removeNull(Map <String,Object>map){
        map.entrySet().removeIf(entry -> entry.getValue() == null);
    }