Java8中使用stream()、filter()、forEach()、collect、distinct、map()

stream方法获取指向当前Collection对象的流对象，filter将对流中元素进行过滤，结合lambda表达式，需要在filter参数中实现一个类似于比较器的Predicate对象，返回一个boolean类型返回值，只有返回为true的Collection中的元素才会进入到forEach的循环中。
 

List strArr = Arrays.asList("21", "22", "3", "4");
 
        strArr.stream().filter(str ->{
            return str.startsWith("2");
        }).filter(str ->{
            return str.equals("22");
        }).forEach(str ->{
            System.out.println(str);
        });

 使用collect将stream转化为list

List result1 = lines.stream()  // convert list to stream
        .filter(line -> !"mkyong".equals(line)) // filter the line which equals to "mkyong"
        .collect(Collectors.toList());  // collect the output and convert streams to a list
 
    result1.forEach(System.out::println); // o

Stream.distinct()  :字符串去重

 List list = Arrays.asList("AA", "BB", "CC", "BB", "CC", "AA", "AA");
        long l = list.stream().distinct().count();
        System.out.println("No. of distinct elements:"+l);
        String output = list.stream().distinct().collect(Collectors.joining(","));
        System.out.println(output);

2. Stream.distinct() with List of Objects

在此示例中，我们有一个Book对象列表。 为了对列表进行去重，该类将重写hashCode（）和equals（）。

public class Book {
    private String name;
    private int price;
    public Book(String name, int price) {
	this.name = name;
	this.price = price;
    }
    public String getName() {
	return name;
    }
    public int getPrice() {
	return price;
    }
    @Override
    public boolean equals(final Object obj) {
      if (obj == null) {
         return false;
      }
      final Book book = (Book) obj;
      if (this == book) {
         return true;
      } else {
         return (this.name.equals(book.name) && this.price == book.price);
      }
    }
    @Override
    public int hashCode() {
      int hashno = 7;
      hashno = 13 * hashno + (name == null ? 0 : name.hashCode());
      return hashno;
    }
} 

List list = new ArrayList<>();
        {
           list.add(new Book("Core Java", 200));
           list.add(new Book("Core Java", 200));
           list.add(new Book("Learning Freemarker", 150));        	
           list.add(new Book("Spring MVC", 300));
           list.add(new Book("Spring MVC", 300));
        }
        long l = list.stream().distinct().count();
        System.out.println("No. of distinct books:"+l);
        list.stream().distinct().forEach(b -> System.out.println(b.getName()+ "," + b.getPrice()));

3. Distinct by Property

distinct（）不提供按照属性对对象列表进行去重的直接实现。它是基于hashCode（）和equals（）工作的。如果我们想要按照对象的属性，对对象列表进行去重，我们可以通过其它方法来实现。如下代码段所示：

static  Predicate distinctByKey(Function keyExtractor) {
        Map seen = new ConcurrentHashMap<>();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
}

上面的方法可以被Stream接口的 filter()接收为参数，如下所示：

list.stream().filter(distinctByKey(b -> b.getName()));

distinctByKey()方法返回一个使用ConcurrentHashMap 来维护先前所见状态的 Predicate 实例，如下是一个完整的使用对象属性来进行去重的示例。

public class DistinctByProperty {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        {
        	list.add(new Book("Core Java", 200));
        	list.add(new Book("Core Java", 300));
        	list.add(new Book("Learning Freemarker", 150));
        	list.add(new Book("Spring MVC", 200));
        	list.add(new Book("Hibernate", 300));
        }
        list.stream().filter(distinctByKey(b -> b.getName()))
              .forEach(b -> System.out.println(b.getName()+ "," + b.getPrice()));   
    }
    private static  Predicate distinctByKey(Function keyExtractor) {
        Map seen = new ConcurrentHashMap<>();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }
}

Foreach 使用（map）

Map items = new HashMap<>();
	items.put("A", 10);
	items.put("B", 20);
	items.put("C", 30);
	items.put("D", 40);
	items.put("E", 50);
	items.put("F", 60);

	for (Map.Entry entry : items.entrySet()) {
		System.out.println("Item : " + entry.getKey() + " Count : " + entry.getValue());
	}


Map items = new HashMap<>();
	items.put("A", 10);
	items.put("B", 20);
	items.put("C", 30);
	items.put("D", 40);
	items.put("E", 50);
	items.put("F", 60);
	
	items.forEach((k,v)->System.out.println("Item : " + k + " Count : " + v));
	
	items.forEach((k,v)->{
		System.out.println("Item : " + k + " Count : " + v);
		if("E".equals(k)){
			System.out.println("Hello E");
		}
	});

map()

https://www.mkyong.com/java8/java-8-streams-map-examples/

public class TestJava8 {

    public static void main(String[] args) {

        List alpha = Arrays.asList("a", "b", "c", "d");

        //Before Java8
        List alphaUpper = new ArrayList<>();
        for (String s : alpha) {
            alphaUpper.add(s.toUpperCase());
        }

        System.out.println(alpha); //[a, b, c, d]
        System.out.println(alphaUpper); //[A, B, C, D]

        // Java 8
        List collect = alpha.stream().map(String::toUpperCase).collect(Collectors.toList());
        System.out.println(collect); //[A, B, C, D]

        // Extra, streams apply to any data type.
        List num = Arrays.asList(1,2,3,4,5);
        List collect1 = num.stream().map(n -> n * 2).collect(Collectors.toList());
        System.out.println(collect1); //[2, 4, 6, 8, 10]

    }

}

2. List of objects -> List of String
2.1 Get all the name values from a list of the staff objects.

Staff.java
package com.mkyong.java8;

import java.math.BigDecimal;

public class Staff {

    private String name;
    private int age;
    private BigDecimal salary;
	//...
}
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TestJava8.java
package com.mkyong.java8;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class TestJava8 {

    public static void main(String[] args) {

        List staff = Arrays.asList(
                new Staff("mkyong", 30, new BigDecimal(10000)),
                new Staff("jack", 27, new BigDecimal(20000)),
                new Staff("lawrence", 33, new BigDecimal(30000))
        );

        //Before Java 8
        List result = new ArrayList<>();
        for (Staff x : staff) {
            result.add(x.getName());
        }
        System.out.println(result); //[mkyong, jack, lawrence]

        //Java 8
        List collect = staff.stream().map(x -> x.getName()).collect(Collectors.toList());
        System.out.println(collect); //[mkyong, jack, lawrence]

    }

}
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3. List of objects -> List of other objects
3.1 This example shows you how to convert a list of staff objects into a list of StaffPublic objects.

Staff.java
package com.mkyong.java8;

import java.math.BigDecimal;

public class Staff {

    private String name;
    private int age;
    private BigDecimal salary;
	//...
}

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StaffPublic.java
package com.mkyong.java8;

public class StaffPublic {

    private String name;
    private int age;
    private String extra;
    //...
}


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3.2 Before Java 8.

BeforeJava8.java
package com.mkyong.java8;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class BeforeJava8 {

    public static void main(String[] args) {

        List staff = Arrays.asList(
                new Staff("mkyong", 30, new BigDecimal(10000)),
                new Staff("jack", 27, new BigDecimal(20000)),
                new Staff("lawrence", 33, new BigDecimal(30000))
        );

        List result = convertToStaffPublic(staff);
        System.out.println(result);

    }

    private static List convertToStaffPublic(List staff) {

        List result = new ArrayList<>();

        for (Staff temp : staff) {

            StaffPublic obj = new StaffPublic();
            obj.setName(temp.getName());
            obj.setAge(temp.getAge());
            if ("mkyong".equals(temp.getName())) {
                obj.setExtra("this field is for mkyong only!");
            }

            result.add(obj);
        }

        return result;

    }

}
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Output

[
	StaffPublic{name='mkyong', age=30, extra='this field is for mkyong only!'}, 
	StaffPublic{name='jack', age=27, extra='null'}, 
	StaffPublic{name='lawrence', age=33, extra='null'}
]
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3.3 Java 8 example.

NowJava8.java
package com.mkyong.java8;

package com.hostingcompass.web.java8;

import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class NowJava8 {

    public static void main(String[] args) {

        List staff = Arrays.asList(
                new Staff("mkyong", 30, new BigDecimal(10000)),
                new Staff("jack", 27, new BigDecimal(20000)),
                new Staff("lawrence", 33, new BigDecimal(30000))
        );

		// convert inside the map() method directly.
        List result = staff.stream().map(temp -> {
            StaffPublic obj = new StaffPublic();
            obj.setName(temp.getName());
            obj.setAge(temp.getAge());
            if ("mkyong".equals(temp.getName())) {
                obj.setExtra("this field is for mkyong only!");
            }
            return obj;
        }).collect(Collectors.toList());

        System.out.println(result);

    }
	
}
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Output

[
	StaffPublic{name='mkyong', age=30, extra='this field is for mkyong only!'}, 
	StaffPublic{name='jack', age=27, extra='null'}, 
	StaffPublic{name='lawrence', age=33, extra='null'}
]