jdk8的特性stream().map()

转:

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 类是一个可以为null的容器对象。如果值存在则isPresent()方法会返回true，调用get()方法会返回该对象。

Optional 是个容器：它可以保存类型T的值，或者仅仅保存null。Optional提供很多有用的方法，这样我们就不用显式进行空值检测。

Optional 类的引入很好的解决空指针异常。

类声明

以下是一个 java.util.Optional<T> 类的声明：

public final class Optional<T> extends Object

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在Java 8中stream().map()，您可以将对象转换为其他对象。查看以下示例：
 
1.大写字符串列表
1.1简单的Java示例将Strings列表转换为大写。
 
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.对象列表 - >字符串列表
2.1 name从staff对象列表中获取所有值。
 
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]
 
    }
 
}
 
对象列表 - >其他对象列表
3.1此示例说明如何将staff对象列表转换为对象列表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之前。
 
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;
 
    }
 
}
 
输出
 
[
    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'}
]