1. Java's own tools and methods
1.1 the list set is spliced into comma separated strings
// How to splice a list set into comma separated strings a,b,c
List<String> list = Arrays.asList("a", "b", "c");
// The first method can use stream
String join = list.stream().collect(Collectors.joining(","));
System.out.println(join); // output a,b,c
// The second method, in fact, String also has a join method to realize this function
String join = String.join(",", list);
System.out.println(join); // output a,b,c
1.2 compare whether two strings are equal, ignoring case
if (strA.equalsIgnoreCase(strB)) {
System.out.println("equal");
}
1.3 compare whether two objects are equal
When we use equals to compare whether two Objects are equal, we also need to judge the null of the object on the left, otherwise a null pointer exception may be reported. We can use the method encapsulated by Objects in java.util package to compare whether they are equal
Objects.equals(strA, strB);
The source code is like this
public static boolean equals(Object a, Object b) {
return (a == b) || (a != null && a.equals(b));
}
1.4 intersection of two List sets
List<String> list1 = new ArrayList<>();
list1.add("a");
list1.add("b");
list1.add("c");
List<String> list2 = new ArrayList<>();
list2.add("a");
list2.add("b");
list2.add("d");
list1.retainAll(list2);
System.out.println(list1); // Output [a, b]
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2. apache commons tool class library
apache commons is the most powerful and widely used tool class library. There are many sub libraries. Here are some of the most commonly used tools
2.1 commons Lang, enhanced version of java.lang
It is recommended to use commons Lang 3. Some APIs have been optimized. The original commons Lang has stopped updating
Maven's dependencies are:
<dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-lang3</artifactId> <version>3.12.0</version> </dependency>
The passed parameter CharSequence type is the parent class of String, StringBuilder and StringBuffer, which can be directly null judged by the following methods. The following is the source code:
public static boolean isEmpty(final CharSequence cs) {
return cs == null || cs.length() == 0;
}
public static boolean isNotEmpty(final CharSequence cs) {
return !isEmpty(cs);
}
// Blank characters in the string, such as spaces, line breaks, and tabs, will be removed during blank judgment
public static boolean isBlank(final CharSequence cs) {
final int strLen = length(cs);
if (strLen == 0) {
return true;
}
for (int i = 0; i < strLen; i++) {
if (!Character.isWhitespace(cs.charAt(i))) {
return false;
}
}
return true;
}
public static boolean isNotBlank(final CharSequence cs) {
return !isBlank(cs);
}
2.1.2 capitalization of initial letters
String str = "yideng"; String capitalize = StringUtils.capitalize(str); System.out.println(capitalize); // Output Yideng
2.1.3 repeated splicing string
String str = StringUtils.repeat("ab", 2);
System.out.println(str); // Output abab
No more handwritten SimpleDateFormat formatting
// Date type to String type
String date = DateFormatUtils.format(new Date(), "yyyy-MM-dd HH:mm:ss");
System.out.println(date); // output 2021-05-01 01:01:01
// String type to Date type
Date date = DateUtils.parseDate("2021-05-01 01:01:01", "yyyy-MM-dd HH:mm:ss");
// Calculate the date after one hour
Date date = DateUtils.addHours(new Date(), 1);
2.1.5 temporary packaging objects
When a method needs to return two or more fields, we usually encapsulate it as a temporary object. Now we don't need it with Pair and Triple
// Return two fields ImmutablePair<Integer, String> pair = ImmutablePair.of(1, "yideng"); System.out.println(pair.getLeft() + "," + pair.getRight()); // output 1,yideng // Return three fields ImmutableTriple<Integer, String, Date> triple = ImmutableTriple.of(1, "yideng", new Date()); System.out.println(triple.getLeft() + "," + triple.getMiddle() + "," + triple.getRight()); // output 1,yideng,Wed Apr 07 23:30:00 CST two thousand and twenty-one
2.2 common collections collection tool class
Maven's dependencies are:
<dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-collections4</artifactId> <version>4.4</version> </dependency>
2.2.1 collective empty judgment
Encapsulates the method of set empty judgment. The following is the source code:
public static boolean isEmpty(final Collection<?> coll) {
return coll == null || coll.isEmpty();
}
public static boolean isNotEmpty(final Collection<?> coll) {
return !isEmpty(coll);
}
// Intersection of two sets
Collection<String> collection = CollectionUtils.retainAll(listA, listB);
// Union of two sets
Collection<String> collection = CollectionUtils.union(listA, listB);
// Difference set of two sets
Collection<String> collection = CollectionUtils.subtract(listA, listB);
2.3 common bean utils operation object
Maven dependency:
<dependency>
<groupId>commons-beanutils</groupId>
<artifactId>commons-beanutils</artifactId>
<version>1.9.4</version>
</dependency>
public class User {
private Integer id;
private String name;
}
Set object properties
User user = new User();
BeanUtils.setProperty(user, "id", 1);
BeanUtils.setProperty(user, "name", "yideng");
System.out.println(BeanUtils.getProperty(user, "name")); // output yideng
System.out.println(user); // output {"id":1,"name":"yideng"}
Object and map rotate with each other
// Object to map
Map<String, String> map = BeanUtils.describe(user);
System.out.println(map); // output {"id":"1","name":"yideng"}
// map to object
User newUser = new User();
BeanUtils.populate(newUser, map);
System.out.println(newUser); // output {"id":1,"name":"yideng"}
2.4 commons IO file stream processing
Maven dependency:
<dependency> <groupId>commons-io</groupId> <artifactId>commons-io</artifactId> <version>2.8.0</version> </dependency>
File processing
File file = new File("demo1.txt");
// read file
List<String> lines = FileUtils.readLines(file, Charset.defaultCharset());
// write file
FileUtils.writeLines(new File("demo2.txt"), lines);
// Copy file
FileUtils.copyFile(srcFile, destFile);
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3. Google Guava tool class library
Maven dependency:
<dependency> <groupId>com.google.guava</groupId> <artifactId>guava</artifactId> <version>30.1.1-jre</version> </dependency>
3.1 creating collections
List<String> list = Lists.newArrayList(); List<Integer> list = Lists.newArrayList(1, 2, 3); // Reverse list List<Integer> reverse = Lists.reverse(list); System.out.println(reverse); // output [3, 2, 1] // There are too many elements in the list set, which can be divided into several sets, with 10 elements in each set List<List<Integer>> partition = Lists.partition(list, 10); Map<String, String> map = Maps.newHashMap(); Set<String> set = Sets.newHashSet();
3.2 black technology collection
3.2.1 Multimap a key can map a HashMap of multiple value s
Multimap<String, Integer> map = ArrayListMultimap.create();
map.put("key", 1);
map.put("key", 2);
Collection<Integer> values = map.get("key");
System.out.println(map); // output {"key":[1,2]}
// It can also return the bloated Map you used before
Map<String, Collection<Integer>> collectionMap = map.asMap();
It's easy and concise to save you from creating map < string, list >
3.2.2 BiMap is a HashMap that cannot duplicate value s
BiMap<String, String> biMap = HashBiMap.create();
// If the value is repeated, the put method throws an exception unless the forcePut method is used
biMap.put("key","value");
System.out.println(biMap); // output {"key":"value"}
// Since value cannot be repeated, why not implement a method to flip key/value? It already exists
BiMap<String, String> inverse = biMap.inverse();
System.out.println(inverse); // output {"value":"key"}
This is actually a two-way mapping, which is very practical in some scenarios.
3.2.3 Table is a HashMap with two key s
// A group of users, grouped by age and gender
Table<Integer, String, String> table = HashBasedTable.create();
table.put(18, "male", "yideng");
table.put(18, "female", "Lily");
System.out.println(table.get(18, "male")); // output yideng
// This is actually a two-dimensional Map, which can view row data
Map<String, String> row = table.row(18);
System.out.println(row); // output {"male": "yideng", "female": "Lily"}
// View column data
Map<Integer, String> column = table.column("male");
System.out.println(column); // output {18:"yideng"}
3.2.4 Multiset is a Set used for counting
Multiset<String> multiset = HashMultiset.create();
multiset.add("apple");
multiset.add("apple");
multiset.add("orange");
System.out.println(multiset.count("apple")); // output two
// View de duplicated elements
Set<String> set = multiset.elementSet();
System.out.println(set); // output ["orange","apple"]
// You can also view elements that have not been de duplicated
Iterator<String> iterator = multiset.iterator();
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
// You can also manually set the number of occurrences of an element
multiset.setCount("apple", 5);