文章目录
- JDK8对List对象根据属性排序
- [1. 被排序字段为null或者空时候报错](#1. 被排序字段为null或者空时候报错)
- [2. 使用Stream流排序](#2. 使用Stream流排序)
-
- [2.1 根据name升序](#2.1 根据name升序)
- [2.2 根据name升序，score降序](#2.2 根据name升序，score降序)
- [3. 使用Collections排序](#3. 使用Collections排序)
-
- [3.1 根据name升序](#3.1 根据name升序)
- [3.2 根据name升序，score降序](#3.2 根据name升序，score降序)
- [4. 完整的demo](#4. 完整的demo)
JDK8对List对象根据属性排序
1. 被排序字段为null或者空时候报错
被排序字段为null或者空的时候报java.lang.NullPointerException
shell
Exception in thread "main" java.lang.NullPointerException
at java.util.Comparator.lambda$comparing$77a9974f$1(Comparator.java:469)
at java.util.TimSort.countRunAndMakeAscending(TimSort.java:356)
at java.util.TimSort.sort(TimSort.java:220)
at java.util.Arrays.sort(Arrays.java:1512)
at java.util.stream.SortedOps$SizedRefSortingSink.end(SortedOps.java:348)
at java.util.stream.Sink$ChainedReference.end(Sink.java:258)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
at com.stormkai.jh.ListSortDemo1.getNameAsc(ListSortDemo1.java:40)
at com.stormkai.jh.ListSortDemo1.main(ListSortDemo1.java:19)使用以下方式处理：
- Comparator.nullsLast：排序字段为null的排在后面
- Comparator.nullsFirst：排序字段为null的排在前面
- 集合工具类Collections
java
Collections.sort(students1,Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo)));- stream流的方式
java
List<Student> students1 = students.stream().map(student -> {
if("".equals(student.getName())){
student.setName(null);
}
return student;
}).sorted(Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo)))
.collect(Collectors.toList())2. 使用Stream流排序
Student.java
java
@Data
@AllArgsConstructor
public class Student {
private Integer id;
private String name;
private double score;
}2.1 根据name升序
java
public class ListSortDemo1 {
public static void main(String[] args) {
List<Student> students = new ArrayList<Student>(){{
add(new Student(1,"张三C",98.50d));
add(new Student(2,"张三B",67.50d));
add(new Student(3,"张三A",85.40d));
add(new Student(4,"",79.60d));
add(new Student(5,null,84.80d));
add(new Student(6,"张三D",95.20d));
}};
List<Student> students1 = getNameAsc(students);
students1.forEach(System.out::println);
}
//按name升序，null和空的name都以null处理，排在最后
private static List<Student> getNameAsc(List<Student> students) {
List<Student> students1 = students.stream().map(student -> {
if("".equals(student.getName())){
student.setName(null);
}
return student;
}).sorted(Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo)))
.collect(Collectors.toList());
return students1;
}
}执行结果：
shell
Student(id=3, name=张三A, score=85.4)
Student(id=2, name=张三B, score=67.5)
Student(id=1, name=张三C, score=98.5)
Student(id=6, name=张三D, score=95.2)
Student(id=4, name=null, score=79.6)
Student(id=5, name=null, score=84.8)2.2 根据name升序，score降序
java
//按name升序，按score降序，null和空的name都以null处理，排在最后
private static List<Student> getNameAscAndScoreDesc(List<Student> students) {
List<Student> students1 = students.stream().map(student -> {
if("".equals(student.getName())){
student.setName(null);
}
return student;
}).sorted(Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo)).reversed().thenComparing(Student::getScore).reversed())
.collect(Collectors.toList());
return students1;
}输出结果：
shell
Student(id=3, name=张三A, score=85.4)
Student(id=2, name=张三B, score=67.5)
Student(id=1, name=张三C, score=98.5)
Student(id=6, name=张三D, score=95.2)
Student(id=5, name=null, score=84.8)
Student(id=4, name=null, score=79.6)3. 使用Collections排序
3.1 根据name升序
java
public class ListSortDemo1 {
public static void main(String[] args) {
List<Student> students = new ArrayList<Student>(){{
add(new Student(1,"张三C",98.50d));
add(new Student(2,"张三B",67.50d));
add(new Student(3,"张三A",85.40d));
add(new Student(4,"",79.60d));
add(new Student(5,null,84.80d));
add(new Student(6,"张三D",95.20d));
}};
//List<Student> students1 = getNameAsc(students);
//List<Student> students1 = getNameAscAndScoreDesc(students);
List<Student> students1 = getNameAsc1(students);
students1.forEach(System.out::println);
}
private static List<Student> getNameAsc1(List<Student> students) {
List<Student> students1 = students.stream().map(student -> {
if ("".equals(student.getName())) {
student.setName(null);
}
return student;
}).collect(Collectors.toList());
Collections.sort(students1,Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo)));
return students1;
}
}输出结果：
shell
Student(id=3, name=张三A, score=85.4)
Student(id=2, name=张三B, score=67.5)
Student(id=1, name=张三C, score=98.5)
Student(id=6, name=张三D, score=95.2)
Student(id=4, name=null, score=79.6)
Student(id=5, name=null, score=84.8)3.2 根据name升序，score降序
java
private static List<Student> getNameAscAndScoreDesc1(List<Student> students) {
List<Student> students1 = students.stream().map(student -> {
if ("".equals(student.getName())) {
student.setName(null);
}
return student;
}).collect(Collectors.toList());
Collections.sort(students1,Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo)).reversed().thenComparing(Student::getScore).reversed());
return students1;
}输出结果：
shell
Student(id=3, name=张三A, score=85.4)
Student(id=2, name=张三B, score=67.5)
Student(id=1, name=张三C, score=98.5)
Student(id=6, name=张三D, score=95.2)
Student(id=5, name=null, score=84.8)
Student(id=4, name=null, score=79.6)4. 完整的demo
java
public class ListSortDemo1 {
public static void main(String[] args) {
List<Student> students = new ArrayList<Student>(){{
add(new Student(1,"张三C",98.50d));
add(new Student(2,"张三B",67.50d));
add(new Student(3,"张三A",85.40d));
add(new Student(4,"",79.60d));
add(new Student(5,null,84.80d));
add(new Student(6,"张三D",95.20d));
}};
//List<Student> students1 = getNameAsc(students);
//List<Student> students1 = getNameAscAndScoreDesc(students);
//List<Student> students1 = getNameAsc1(students);
List<Student> students1 = getNameAscAndScoreDesc(students);
students1.forEach(System.out::println);
}
//按name升序，null和空的name都以null处理，排在最后
private static List<Student> getNameAsc(List<Student> students) {
List<Student> students1 = students.stream().map(student -> {
if("".equals(student.getName())){
student.setName(null);
}
return student;
}).sorted(Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo)))
.collect(Collectors.toList());
return students1;
}
private static List<Student> getNameAsc1(List<Student> students) {
List<Student> students1 = students.stream().map(student -> {
if ("".equals(student.getName())) {
student.setName(null);
}
return student;
}).collect(Collectors.toList());
Collections.sort(students1,Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo)));
return students1;
}
//按name升序，按score降序，null和空的name都以null处理，排在最后
private static List<Student> getNameAscAndScoreDesc(List<Student> students) {
List<Student> students1 = students.stream().map(student -> {
if("".equals(student.getName())){
student.setName(null);
}
return student;
}).sorted(Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo)).reversed()
.thenComparing(Student::getScore).reversed())
.collect(Collectors.toList());
return students1;
}
private static List<Student> getNameAscAndScoreDesc1(List<Student> students) {
List<Student> students1 = students.stream().map(student -> {
if ("".equals(student.getName())) {
student.setName(null);
}
return student;
}).collect(Collectors.toList());
Collections.sort(students1,Comparator.comparing(Student::getName,Comparator.nullsLast(String::compareTo)).reversed()
.thenComparing(Student::getScore).reversed());
return students1;
}
}