Right click on the project in which you want to put jar file. A window will open like this

Click on the AddExternal Jars there you can give the path to that jar file

• Right click on the Eclipse project,
• choose Properties
• Select Java Build Path
• Click the libraries tab
• click add external jars
• find the json jar(s) and add them.

You need to use the fully-qualified class name, which includes the package:

import org.json.JSONArray;

(And yes, that is the correct way to add classes/jars to your project's classpath in Eclipse.)

Update for latest version

Pre-requisite: In VS Code you need to open the folder and not just file to include any jar file for compilation, running, debugging and testing

VSCode works fine with maven or gradle, and if we are using any of those, we should use maven or gradle for dependency management.

If you are not using any dependency management, then there are two options to add jar file

Option 1: Manually

Open .vscode/settings.json file (if not present, you can create one) and add required jar file there as shown below. In this case all the jar files present in lib directory will be included + I have added common-logging-1.1.1.jar which is located outside the project directory

{
    "java.project.referencedLibraries": [
        "lib/**/*.jar",
        "c:\\path\\to\\jarfile\\commons-logging-1.1.1.jar"
    ]
}

Option 2: Via User Interface

If Java Extension is enabled then on left panel there will be Java Dependencies. Inside there you can click on + symbol to add customer jar files. With this method too, the settings.json file will be updated

Old Answer kept for reference...

VSCode works fine with maven or gradle. But without those, as far as I know they have not provided any direct way to add jar file in classpath.

The workaround is to add the entry in .classpath file manually.

For e.g. in below file I have added common-logging-1.2.jar located in lib directory manually by adding a line <classpathentry exported="true" kind="lib" path="lib/commons-logging-1.2.jar"/>

<?xml version="1.0" encoding="UTF-8"?>
<classpath>
        <classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-1.8"/>
        <classpathentry kind="src" path="src"/>
        <classpathentry kind="output" path="bin"/>
        <classpathentry exported="true" kind="lib" path="lib/commons-logging-1.2.jar"/>
</classpath>

Update

Finally VSCode team is working towards a better solution. This is still work in progress, but you can refer to this link for better aproach https://github.com/microsoft/vscode-java-pack/issues/94#issuecomment-573487982

Update April 2020

VSCode Extension update now supports this feature out of the box. Now VS Code Java lets you work with JAR files directly without any build tools. Go to JAVA DEPENDENCIES view, find the Referenced Libraries node and click the + icon.

Reference https://github.com/microsoft/vscode-java-pack/blob/master/release-notes/v0.9.0.md#work-with-jar-files-directly

You have to use the -classpath when running with java as well.

java -classpath json.jar MyClassName

If using Maven, add this dependency to your pom.xml

<dependency>
    <groupId>javax.json</groupId>
    <artifactId>javax.json-api</artifactId>
    <version>1.0</version>
</dependency>

For Gradle, add this to your build.gradle

compile 'javax.json:javax.json-api:1.0'

The question is a bit unclear, so my answer is equally general...

In order to use a third-party library in Java (like JSON-lib in your case), the library must be present during compilation and during execution (runtime) of your program. This is done by downloading the .jar file and telling Java where to find it. Java uses the concept of classpath:

Classpath is a parameter – set either on the command-line, or through an environment variable – that tells the Java Virtual Machine or the compiler where to look for user-defined classes and packages.

(from Wikipedia)

The error you mention ("Import org.json.JSONObject cannot be resolved") means that the .jar wasn't on the classpath during compilation.

To add a third-party library in Eclipse, store the .jar file somewhere in your project (a dedicated lib folder would be a good choice), then right-click on your project, choose Properties → Java Build Path → Libraries, click "Add JARs..." and choose your .jar file. From now on, Eclipse will add it to the classpath during compilation.

Adding it to the classpath during execution depends on the kind of project you're running. If you have a simple main method which you're running from Eclipse, the .jar will be automatically added to the classpath. If you're running a web application in a container such as Tomcat, JBoss etc., then the standard way to add the .jar to the classpath would be to put it in the WEB-INF/lib directory of your deployed application. Have a look at the Java EE tutorial for more information on web applications and their deployment.