How Is Instantiation Different from Initialization of a Java Variable?

Variables in Java store data (e.g., a string or number). How is instantiation of a variable different from initialization, or are they the same?

Instantiation is different from initialization. Instantiation is the creation of the variable (e.g., through the Java syntax or invocation of a method that creates the variable). A class can be instantiated or a primitive variable can be instantiated.

How Do You Troubleshoot “/usr/bin/java no such file or directory”?

Problem scenario
You run java commands but you get the error “/usr/bin/java no such file or directory.” What should you do?

Possible Solution #1
Reinstall Java.

Possible Solution #2
You may have deleted a linked file. To find the destination file, run this:

sudo find / -name java -type f

Based on the results,

How Do You Run Code in the Eclipse IDE?

Problem scenario
You are using Eclipse for the first time. You try to to use Run, but you get an error message like this: “Unable to Launch The selection cannot be launched, and there are no recent launches.” Or you try to go to “Run configuration” and you have a prototype, but the “Run” button is grayed out. What should you do?

In the Run configuration section,

How Do You Troubleshoot dpkg Errors from apt Commands like “package post-installation script subprocess returned error exit status 1”?

Problem scenario
On a Debian/Ubuntu server, you are trying to run apt commands. But you get errors like this:

dpkg: error processing package oracle-java11-installer-local (–configure):
installed oracle-java11-installer-local package post-installation script subprocess returned error exit status 1
Setting up nmap-common (7.80+dfsg1-2build1) …
Setting up liblua5.3-0:amd64 (5.3.3-1.1ubuntu2) …
dpkg: dependency problems prevent configuration of default-jre-headless:
default-jre-headless depends on openjdk-11-jre-headless; however:
Package openjdk-11-jre-headless is not installed.

How Do You Troubleshoot Maven Giving You an Error Such as “java.lang.OutOfMemoryError java heap space”?

Problem scenario
You are getting an error such as “java.lang.OutOfMemoryError java heap space”

You have plenty of swap space available, but this error persists. The problem is as if nothing is using swap space you know exists. Why won’t your Java program or Maven build utilize the swap space that is available to it?

Possible solution #1 (for Maven builds)
Run a command like one of these (but uncomment the second one if you want to commit roughly 5 GB of memory to the maven build):

export MAVEN_OPTS=-Xmx512m
# export MAVEN_OPTS=-Xmx5120m

Now retry your mvn command.

How Do You Solve the MacOS Command Line Error “No Java runtime present…”?

Problem scenario
Using a MacOS, you are trying to run a command (e.g., a “java” command) from the Terminal utility (character prompt). You get the message “No Java runtime present”, and the command fails. What should you do?

Using the GUI, download the .dmg file from here.

Once it is downloaded, double click it. Follow the menu steps (Continue ->

How Do You Write a Java Program to Accept a Character as Input and Do Something with It?

Problem scenario
You want to accept user input with a Java program. How do you write such a program?

Here is the program:

import java.util.Scanner;

public class acceptChar {

public static void main(String[] args) {
System.out.println(“Please enter a character, then press enter: “);
Scanner in = new Scanner(;
char letter =;

How Do You Get Presto to Start when Errors about Presto requiring an Oracle or OpenJDK JVM?

Problem scenario
Presto will not run because of an error about the JVM.

You get a message like this:
“Java version has an unknown format: 15” (from “C” below)
“Presto requires an Oracle or OpenJDK JVM (found Ubuntu)” (associated with the Java version shown in B or D below)
“Presto requires an Oracle or OpenJDK JVM (found Private Build)” (associated with the Java version in E below)
“Presto requires an Oracle or OpenJDK JVM (found IcedTea)” (associated with the Java version shown in F below)

For detailed background information,