CollectionTools 新增 nullToEmptyList、nullToEmptySet、nullToEmptyMap 方法

Reviewed-on: http://zhouxy.xyz:3000/plusone/plusone-commons/pulls/10

src/main/java/xyz/zhouxy/plusone/commons/collection/CollectionTools.java

@@ -17,8 +17,12 @@
 package xyz.zhouxy.plusone.commons.collection;
 
 import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
 import java.util.Map;
+import java.util.Set;
 
+import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 
 /**
@@ -51,6 +55,21 @@ public class CollectionTools {
         return map != null && !map.isEmpty();
     }
 
+    @Nonnull
+    public static <T> List<T> nullToEmptyList(@Nullable List<T> list) {
+        return list == null ? Collections.emptyList() : list;
+    }
+
+    @Nonnull
+    public static <T> Set<T> nullToEmptySet(@Nullable Set<T> set) {
+        return set == null ? Collections.emptySet() : set;
+    }
+
+    @Nonnull
+    public static <K, V> Map<K, V> nullToEmptyMap(@Nullable Map<K, V> map) {
+        return map == null ? Collections.emptyMap() : map;
+    }
+
     private CollectionTools() {
         throw new IllegalStateException("Utility class");
     }