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]

2. apache commons tool class library

apache commons It is the most powerful and widely used tool class library. There are many sub libraries. Here are some of the most commonly used

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>

2.1.1 empty string

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

2.1.4 format date

No more handwritten SimpleDateFormat formatting

// Date Type conversion 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 conversion 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 2021

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-beanutils   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 rotation map
Map<String, String> map = BeanUtils.describe(user);
System.out.println(map); // output {"id":"1","name":"yideng"}
// map Transfer 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);

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);
// reversal list
List<Integer> reverse = Lists.reverse(list);
System.out.println(reverse); // output [3, 2, 1]
// list There are too many set elements, 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 also returns the bloated you used before Map
Map<String, Collection<Integer>> collectionMap = map.asMap();

How easy and concise it is to save you from creating it again   Map<String, List<Integer>>

3.2.1 BiMap is a HashMap that cannot duplicate value s

BiMap<String, String> biMap = HashBiMap.create();
// If value Repeat, put Method throws exceptions unless forcePut method
biMap.put("key","value");
System.out.println(biMap); // output {"key":"value"}
// since value Can't repeat, why don't you realize a flip key/value There is already a way
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，You 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 2
// 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);