Java8 对 Map 排序

引言

使用 keys 或 values 对 map 排序。

1. 快速开始

步骤：

将 map 转为流对流排序收集并返回一个新的 LinkedHashMap (保持顺序) Map result = map.entrySet().stream() .sorted(Map.Entry.comparingByKey()) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new));

默认情况下，Collectors.toMap 将返回一个 HashMap。

2. 按 Keys 排序

public static void main(String[] args) { Map<String, Integer> unsortMap = new HashMap<>(); unsortMap.put("z", 10); unsortMap.put("b", 5); unsortMap.put("a", 6); unsortMap.put("c", 20); unsortMap.put("d", 1); unsortMap.put("e", 7); unsortMap.put("y", 8); unsortMap.put("n", 99); unsortMap.put("g", 50); unsortMap.put("m", 2); unsortMap.put("f", 9); System.out.println("Original..."); System.out.println(unsortMap); // sort by keys, a,b,c..., and return a new LinkedHashMap // toMap() will returns HashMap by default, we need LinkedHashMap to keep the order. LinkedHashMap<String, Integer> result = unsortMap.entrySet().stream() .sorted(Entry.comparingByKey()).collect(Collectors .toMap(Entry::getKey, Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new)); // 不推荐，但是很有效。 Map<String, Integer> result2 = new LinkedHashMap<>(); unsortMap.entrySet().stream().sorted(Map.Entry.comparingByKey()) .forEachOrdered(x->result2.put(x.getKey(),x.getValue())); System.out.println("Sorted..."); System.out.println(result); System.out.println(result2); }

输出：

Original... {a=6, b=5, c=20, d=1, e=7, f=9, g=50, y=8, z=10, m=2, n=99} Sorted... {a=6, b=5, c=20, d=1, e=7, f=9, g=50, m=2, n=99, y=8, z=10} {a=6, b=5, c=20, d=1, e=7, f=9, g=50, m=2, n=99, y=8, z=10}

3. 按 Values 排序

public static void main(String[] args) { Map<String, Integer> unsortMap = new HashMap<>(); unsortMap.put("z", 10); unsortMap.put("b", 5); unsortMap.put("a", 6); unsortMap.put("c", 20); unsortMap.put("d", 1); unsortMap.put("e", 7); unsortMap.put("y", 8); unsortMap.put("n", 99); unsortMap.put("g", 50); unsortMap.put("m", 2); unsortMap.put("f", 9); System.out.println("Original..."); System.out.println(unsortMap); //sort by values, and reserve it, 10,9,8,7,6... LinkedHashMap<String, Integer> result = unsortMap.entrySet().stream() .sorted(Entry.comparingByValue(Comparator.reverseOrder())) .collect(Collectors.toMap(Entry::getKey, Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new)); // 替代方式 Map<String, Integer> result2 = new LinkedHashMap<>(); unsortMap.entrySet().stream() .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder())) .forEachOrdered(x -> result2.put(x.getKey(), x.getValue())); System.out.println("Sorted..."); System.out.println(result); System.out.println(result2); }

输出：

Original... {a=6, b=5, c=20, d=1, e=7, f=9, g=50, y=8, z=10, m=2, n=99} Sorted... {n=99, g=50, c=20, z=10, f=9, y=8, e=7, a=6, b=5, m=2, d=1} {n=99, g=50, c=20, z=10, f=9, y=8, e=7, a=6, b=5, m=2, d=1}

4. Map<Object,Object>

Stream 不能直接对 Map<Object,Object> 进行排序，可以现将它转为 Map<String,String>，再操作：

public static void main(String[] args) { Properties properties = System.getProperties(); Set<Entry<Object, Object>> entries = properties.entrySet(); LinkedHashMap<String, String> collect = entries.stream() .collect(Collectors.toMap(k -> (String) k.getKey(), e -> (String) e.getValue())) .entrySet() .stream().sorted(Entry.comparingByKey()) .collect(Collectors .toMap(Entry::getKey, Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new)); collect.forEach((k,v)->System.out.println(k + ":" + v)); }

输出：

awt.toolkit:sun.lwawt.macosx.LWCToolkit file.encoding:UTF-8 file.encoding.pkg:sun.io file.separator:/ ftp.nonProxyHosts:local|*.local|169.254/16|*.169.254/16 gopherProxySet:false http.nonProxyHosts:local|*.local|169.254/16|*.169.254/16 java.awt.graphicsenv:sun.awt.CGraphicsEnvironment java.awt.printerjob:sun.lwawt.macosx.CPrinterJob ..... sun.boot.library.path:/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/Contents/Home/jre/lib sun.cpu.endian:little sun.cpu.isalist: sun.io.unicode.encoding:UnicodeBig sun.java.command:com.jimzhang.map.sort.ObjectTest sun.java.launcher:SUN_STANDARD sun.jnu.encoding:UTF-8 sun.management.compiler:HotSpot 64-Bit Tiered Compilers sun.os.patch.level:unknown user.country:CN user.dir:/Users/zhangjinmiao/Documents/GitHub/java-8-demo user.home:/Users/zhangjinmiao user.language:zh user.name:zhangjinmiao user.timezone:

源码见：java-8-demo

系列文章详见：Java 8 教程

转载于:https://www.cnblogs.com/itzjm/articles/11454843.html