Quickly implement a singleton with an enum in Java

In the past, I used to use Initialize-On-Demand Holder, but recently I have made a singleton with enum. Below is an example of a string utility class.

Implementation example

Basically, create an enum that also has the only element, ʻINSTANCE`, All you have to do now is create the public method normally. It's easy and easy.


package jp.fumokmm;

import java.util.Objects;

public enum StringUtility {

	 * <p>Trims the specified string.
	 *If the specified string is null, an empty string will be returned.</p>
	 * @param str string to trim
	 * @return String after trimming
	public String trim(String str) {
		return Objects.nonNull(str) ? str.trim() : "";

	 * <p>Trims the specified character string (including double-byte spaces).
	 *If the specified string is null, an empty string will be returned.</p>
	 * @param str string to trim
	 * @return String after trimming
	public String trimFull(String str) {
		if (Objects.isNull(str)) return "";
		int len = str.length();
		int st = 0;
		char[] val = str.toCharArray();
		while (st < len && (val[st] <= ' ' || val[st] == ' ')) {
		while (st < len && (val[len - 1] <= ' ' || val[len - 1] == ' ')) {
		return (st > 0 || len < str.length()) ? str.substring(st, len) : str;


Usage example


package jp.fumokmm;

import static org.hamcrest.CoreMatchers.*;
import static org.junit.Assert.*;

import org.junit.Test;

public class StringUtilityTest {

	public void testTrim() {
		StringUtility util = StringUtility.INSTANCE;
		assertThat(util.trim(null), is(""));
		assertThat(util.trim(" "), is(""));
		assertThat(util.trim(" a "), is("a"));
		assertThat("Full-width space is not trimmed", util.trim("  a  "), is("  a  "));

	public void testTrimFull() {
		StringUtility util = StringUtility.INSTANCE;
		assertThat(util.trimFull(null), is(""));
		assertThat(util.trimFull(" "), is(""));
		assertThat(util.trimFull(" a "), is("a"));
		assertThat("Full-width space is also trimmed", util.trimFull("  a  "), is("a"));


By the way, in the above test

StringUtility util = StringUtility.INSTANCE;

I use it after assigning it to a variable once like


You can use it as it is. Please check the readability and use it.

that's all.

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