Java cheasheet

less than 1 minute read

Simple pom.xml

<project>
  <modelVersion>4.0.0</modelVersion>
 
  <groupId>com.mycompany.app</groupId>
  <artifactId>my-app</artifactId>
  <version>1.0-SNAPSHOT</version>
</project>

Maven

mvn exec:java -Dexec.mainClass="Main" # => Run java Main.java from maven.
System.out.printf("%3d    ", n); // print number with spaces.
System.out.println(Files.readAllLines(Paths.get("file.txt"))); // Read file with java.
Arrays.copyOfRange([1,2,3], 1, [1,2,3].length);  // subarray from 1 to length array

Initialize 2 dim array

int[][] input = new int[][] {
    { 1, 2, 3, 4 },
    { 2, 2, 3, 2 }
};

print two dim array

java.util.Arrays.deepToString(table)

Read stdin

Scanner sc = new Scanner(System.in);
while (sc.hasNextLine()) {
    System.out.println(sc.nextLine());
} 

spaced delimeted string to stream

import java.util.stream.*;
import java.util.regex.*;

Stream stream = Pattern.compile(" ").splitAsStream(scanner.nextLine()); // O(1)

// or simpler but would read the string completely and split to array // O(2)

Arrays.stream(sc.nextLine().split(" ")).forEach(item -> {});
// calculate total sum with streams.
int totalSum = Arrays.stream(lineAsStr.split(" "))
            .map(numAsStr -> Integer.valueOf(numAsStr))
            .reduce(0, (sum, n) -> sum += n);

Ordered Tree Map

As it sounds it’s a map so you put things but you can traverse keys in ordered way.

		TreeMap treeMap = new TreeMap();

		treeMap.put("a", "b");
		treeMap.put("d", "d");
		treeMap.put("c", "c");

		Iterator it = treeMap.entrySet().iterator();
		while (it.hasNext()) {
			System.out.println(it.next()); // Printed by key order.
		}

Resources

  1. Java Stream Tutorial

Categories: , ,

Updated:

Leave a Comment