After working for many years, I found that there are many tool class libraries that can greatly simplify the amount of code and improve development efficiency, but junior developers don't know. These class libraries have long become the industry standard class libraries, and they are also used in large companies. If someone told me to use these tool class libraries when I first started working, how good it would be!
Take a look at the tool class libraries you have also used.
1. Java comes with tools method
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 we may report a null pointer exception. We can use Java The method encapsulated by Objects under 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-langļ¼java. Enhanced version of 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 when blank is determined
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 capitalize initial
String str = "yideng"; String capitalize = StringUtils.capitalize(str); System.out.println(capitalize); // Output Yideng
2.1. 3 repeat splice 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 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 packaging temporary 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 set 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);
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. 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 can't even 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 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);
The above is personal experience. I hope I can give you a reference. If there are mistakes or incomplete considerations, please don't hesitate to give me advice.
Author: one lamp architecture
Source: www.toutiao.com com/i6943239541448917512
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Article source: https://www.cnblogs.com/javastack/p/15098747.html