jdk8的特性stream().map()

轉:

https://blog.csdn.net/sanchan/article/details/70753645

java8的optional的使用：

http://www.jdon.com/idea/java/using-optional-effectively-in-java-8.html

http://www.runoob.com/java/java8-optional-class.html

Optional 類是一個能夠爲null的容器對象。若是值存在則isPresent()方法會返回true，調用get()方法會返回該對象。

Optional 是個容器：它能夠保存類型T的值，或者僅僅保存null。Optional提供不少有用的方法，這樣咱們就不用顯式進行空值檢測。

Optional 類的引入很好的解決空指針異常。

類聲明

如下是一個 java.util.Optional<T> 類的聲明：

public final class Optional<T> extends Object

在Java 8中stream().map()，您能夠將對象轉換爲其餘對象。查看如下示例：

1.大寫字符串列表
1.1簡單的Java示例將Strings列表轉換爲大寫。

TestJava8.java

package com.mkyong.java8;

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<String> alpha = Arrays.asList("a", "b", "c", "d");

        //Before Java8
        List<String> 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<String> 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<Integer> num = Arrays.asList(1,2,3,4,5);
        List<Integer> collect1 = num.stream().map(n -> n * 2).collect(Collectors.toList());
        System.out.println(collect1); //[2, 4, 6, 8, 10]

    }

}

2.對象列表 - >字符串列表
2.1 name從staff對象列表中獲取全部值。

Staff.java

package com.mkyong.java8;

import java.math.BigDecimal;

public class Staff {

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

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

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

    }

}

對象列表 - >其餘對象列表
3.1此示例說明如何將staff對象列表轉換爲對象列表StaffPublic。

Staff.java

package com.mkyong.java8;

import java.math.BigDecimal;

public class Staff {

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

StaffPublic.java

package com.mkyong.java8;

public class StaffPublic {

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

3.2 Java 8以前。

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> 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<StaffPublic> result = convertToStaffPublic(staff);
        System.out.println(result);

    }

    private static List<StaffPublic> convertToStaffPublic(List<Staff> staff) {

        List<StaffPublic> 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;

    }

}

輸出

[
    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的例子。

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> 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<StaffPublic> 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);

    }

}


[
    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'}
]