检查方法调用的前置条件
一般尽快失败来拒绝不符合条件的调用
private void addAge(User user,int age){
//检查非空， throw NullPointerException
Preconditions.checkNotNull(user,"%s must not be null","user");
//检查参数，throw IllegalArgumentException
Preconditions.checkArgument(age>0,"the %s should greater than 0; now: %s","age",age);
//检查对象的状态,throw IllegalStateException
Preconditions.checkState(user.getAge()>10,"the age of %s should larger than 10; now: %s",user.getName(),2);
//检查index作为索引值对某个列表、字符串或数组是否有效。 throw IndexOutOfBoundsException
List<String> list = new ArrayList<>();
int size = list.size();
for (int i = 0; i < size; i++) {
Preconditions.checkElementIndex(i,size);
Preconditions.checkPositionIndex(i,size);
}
}
集合
| 支持空元素集合 | 不支持空元素集合 |
|---|---|
| JDK 中的集合很多都支持空元素（ArrayList，LinkedList, HashSet, HashMap） | EnumMap，ConcurrentHashMap 和 guava中的集合大多不支持空元素 |
转换成不可变集合
guava方式 比 Collections.unmodifiableXXX方法 更高效和安全;
最好使用 static final 显示声明，更明显
//使用of 方法直接构建不可变集合
public static final ImmutableSet<String> COLOR_NAMES = ImmutableSet.of(
"red","orange","yellow","green","blue","purple");
//使用copyOf 转换成不可变集合
ImmutableList<String> immutableList = ImmutableList.copyOf(Collection);
ImmutableSet immutableSet = ImmutableSet.copyOf(Collection);
ImmutableMap<Integer,Integer> immutableMap = ImmutableMap.copyOf(Map);
| jdk可变集合接口 | guava中不可变版本 |
|---|---|
| Collection | ImmutableCollection |
| List | ImmutableList |
| Set | ImmutableSet |
| SortedSet/NavigableSet | ImmutableSortedSet |
| Map | ImmutableMap |
| SortedMap | ImmutableSortedMap |
新集合类型
Multiset
可以多次添加相等的元素 统计出现的次数；
Multiset<String> set = HashMultiset.create();
//Multiset<String> set = TreeMultiset.create();
//Multiset<String> set = LinkedHashMultiset.create();
//Multiset<String> set = ConcurrentHashMultiset.create();
//Multiset<String> set = ImmutableMultiset.copyOf(set);
set.add("a");
set.add("b");
set.setCount("c",101);
System.out.println(set.count("c")); //101
System.out.println(set.size()); //103
Multimap
允许key重复
//注意Multimap<K, V>不是Map<K,Collection<V>>，里面的key是有重复的，但entry（key & value）不能都一样
Multimap multimap = HashMultimap.create();
//Multimap multimap = TreeMultimap.create();
//Multimap multimap = LinkedListMultimap.create();
//ImmutableMultimap.copyOf(multimap);
multimap.put("1","2");
multimap.put("4","22");
multimap.put("13",null);
multimap.put("13","23");
multimap.put("13","23");
multimap.put("2","2");
System.out.println(multimap.get("13").size()); //2
BiMap
key和value都唯一，双向绑定，可以反转 key-value
BiMap<String,Integer> bitMap = HashBiMap.create();
//TreeMultimap.create();
//BiMap<DemoEnum,OtherEnum> bitMap2 = EnumBiMap.create(DemoEnum.class,OtherEnum.class);
bitMap.put("a",97);
bitMap.put("b",98);
bitMap.put("c",99);
BiMap<Integer,String> inverseMap = bitMap.inverse();
System.out.println(bitMap.get("a")); //97
System.out.println(inverseMap.get(97)); //'a'
table
用"列"和"行" 组合做key
//泛型参数分别是：列，行，值
HashBasedTable<Integer,Integer,String> table = HashBasedTable.create();
//TreeBasedTable.create();
//ImmutableTable.copyOf(table);
table.put(1,1,"1");
table.put(1,2,"2");
table.put(1,3,"3");
table.put(2,1,"4");
table.put(2,2,"5");
table.put(2,3,"6");
Map<Integer,String> map1 = table.row(2); //检索第二列 {1=4, 2=5, 3=6}
Map<Integer,String> map2 = table.column(1); //检索第一行 {1=1, 2=4}
String value = table.get(1,3); //3
ClassToInstanceMap
它的键是类型，而值是符合键所指类型的对象
//泛型参数代表Map支持的类型的上界
MutableClassToInstanceMap<Number> map = MutableClassToInstanceMap.create();
//ImmutableClassToInstanceMap.copyOf(map);
map.put(Integer.class,1);
map.put(Long.class,101L);
map.put(BigDecimal.class,BigDecimal.ONE);
map.put(short.class,Short.MAX_VALUE);
System.out.println(map.getInstance(short.class)); //32767
RangeSet,TreeRangeMap
不相连的、非空的区间
//使用Comparable 比较
//RangeSet会合并区间，RangeMap不会合并区间
RangeSet<Integer> rangeSet = TreeRangeSet.create();
rangeSet.add(Range.closedOpen(0,10));
rangeSet.add(Range.closed(10,20));
rangeSet.add(Range.openClosed(21,30));
System.out.println(rangeSet.toString()); //[[0..20], (21..30]
TreeRangeMap<String,String> treeRangeMap = TreeRangeMap.create();
treeRangeMap.put(Range.closed("a","f"),"a");
treeRangeMap.put(Range.closed("h","z"),"b");
treeRangeMap.remove(Range.closed("c","d"));
System.out.println(treeRangeMap.toString()); //[[a..c)=a, (d..f]=a, [h..o]=b]