Java Packages & Import
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Standard Class Imports
A standard class import (import java.util.ArrayList;) makes one specific class available by its short name in the current file, without needing to write its fully qualified path every time it's used.
Example: Standard Class Imports
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<String> list = new ArrayList<>(); // short name, no full path needed
list.add("a");
System.out.println(list);
}
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Wildcard Package Imports
A wildcard import (import java.util.*;) brings in every public class from a package at once, which is convenient but can obscure exactly which classes a file actually depends on.
Example: Wildcard Package Imports
import java.util.*;
public class Main {
public static void main(String[] args) {
List<String> list = new ArrayList<>(); // brings in every public class from java.util
list.add("a");
System.out.println(list);
}
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Static Imports
A static import (import static java.lang.Math.PI;) lets you reference a class's static member directly by name (PI) instead of qualifying it every time (Math.PI), which is handy for frequently used constants or methods.
Example: Static Imports
import static java.lang.Math.PI;
public class Main {
public static void main(String[] args) {
System.out.println(PI); // no Math. prefix needed
}
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Using Fully Qualified Names
You can always skip importing entirely and reference a class by its fully qualified name (java.util.List<String>), which is occasionally necessary to disambiguate two classes with the same simple name from different packages.
Example: Using Fully Qualified Names
public class Main {
public static void main(String[] args) {
java.util.List<String> list = new java.util.ArrayList<>(); // no import at all
list.add("a");
System.out.println(list);
}
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Implicitly Imported Packages
Classes in java.lang (like String, Object, and System) are available in every Java file automatically without any import statement, since that package is implicitly imported everywhere.
Example: Implicitly Imported Packages
public class Main {
public static void main(String[] args) {
String s = "Hello"; // java.lang.String, no import needed
System.out.println(s);
}
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