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
public final class Optional extends Object
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 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.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 = 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]
}
}
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 = 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;
}
}
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'}
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 = 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);
}
}
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'}