根据List中的对象的某个属性，求和

我会带着你远行 2024-03-26 16:43 63阅读 0赞

一、根据List中的对象的某个属性，求和

double mathAverageInt = list.stream().mapToInt( Student::getMathScoresInt ).sum().orElse(0d);
    double mathAverageLong = list.stream().mapToLong( Student::getMathScoresLong ).sum().orElse(0d);
    double mathAverageDouble = list.stream().mapToDouble( Student::getMathScoresDouble ).sum().orElse(0d);
    BigDecimal kczl = entities.stream().map(BloodRkdEntity::getKcxl).reduce(BigDecimal.ZERO, BigDecimal::add);

二、根据List中的对象的某个属性，求平均值

double mathAverageInt = list.stream().mapToInt( Student::getMathScoresInt ).average().orElse(0d);
    double mathAverageLong = list.stream().mapToLong( Student::getMathScoresLong ).average().orElse(0d);
    double mathAverageDouble = list.stream().mapToDouble( Student::getMathScoresDouble ).average().orElse(0d);

三、根据List中的对象的某个属性，求最大值

int mathMaxInt = list.stream().mapToInt( Student::getMathScoresInt ).max().getAsInt(); //int类型
    long mathMaxLong = list.stream().mapToLong( Student::getMathScoresLong ).max().getAsLong();
    double mathMaxDouble = list.stream().mapToDouble( Student::getMathScoresDouble ).max().getAsDouble();
    BigDecimal mathMaxBigDecimal = list.stream().map( Student::getMathScoresBigDecimal ).reduce(BigDecimal.ZERO, BigDecimal::max);

四、根据List中的对象的某个属性，求最小值

int mathMinInt = list.stream().mapToInt( Student::getMathScoresInt ).min().getAsInt();
    long mathMinLong = list.stream().mapToLong( Student::getMathScoresLong ).min().getAsLong();
    double mathMinDouble = list.stream().mapToDouble( Student::getMathScoresDouble ).min().getAsDouble();
    BigDecimal mathMinBigDecimal = list.stream().map( Student::getMathScoresBigDecimal ).reduce(BigDecimal.ZERO, BigDecimal::min);

**示例**  
代码：

public static void main(String[] args) throws Exception {
        
        List<Pool> list = new ArrayList<Pool>(){
        
          {
        
            add(new Pool("A", 1));
            add(new Pool("A", 2));
            add(new Pool("A", 3));
            add(new Pool("B", 4));
            add(new Pool("B", 5));
          }
        };
    
        // 求和
        int sum = list.stream().mapToInt(Pool::getValue).sum();
        // 最大值
        OptionalInt max = list.stream().mapToInt(Pool::getValue).max();
        // 最小值
        OptionalInt min = list.stream().mapToInt(Pool::getValue).min();
        // 平均值
        OptionalDouble average = list.stream().mapToInt(Pool::getValue).average();
    
        System.err.println(sum);
        System.err.println(max.getAsInt());
        System.err.println(min.getAsInt());
        System.err.println(average.getAsDouble());
    
      }

输出结果：

15
    5
    1
    3.0

其他方法：  
当然，Stream远不止这么简单，不止有mapToInt()，还有mapToLong()、mapToDouble()等方法。

list.stream().mapToLong(Pool::getValue).sum();
    list.stream().mapToLong(Pool::getValue).max();
    list.stream().mapToLong(Pool::getValue).min();
    list.stream().mapToLong(Pool::getValue).average();
    
    
    list.stream().mapToDouble(Pool::getValue).sum();
    list.stream().mapToDouble(Pool::getValue).max();
    list.stream().mapToDouble(Pool::getValue).min();
    list.stream().mapToDouble(Pool::getValue).average();