VS Code does not have built-in support for Java projects. You need to install specific Java extensions and configure them to specify the correct Java JDK version. Depending on the extension, you can specify the version by either setting the JAVA_HOME environment variable or by setting the java.home setting:

After installing the JDK, you would need to configure your environment for Java development. The most common way is to set JAVA_HOME environment variable to the install location of the JDK while you can also use java.home setting in Visual Studio Code settings (workspace or user settings) to configure it just for the editor.

I described the installation/setup steps in more detail below. It is mostly based on the Visual Studio Code tutorial for Java. Since the OP commented that they are using macOS, this answer is focused on the Java extension for VS Code on Mac. Also, note that depending on the version of the JDK you have installed, java.home might already be deprecated or has been completely removed, and java.jdt.ls.java.home should be used instead (source). So, replace accordingly. Lekia's comment provides a tip:

you can find java properties including java.home from terminal java -XshowSettings:properties -version

MacOS

First, you need to install the Microsoft Java Extension Pack.

Upon installation, it will display the Java Overview tab, and automatically check for available Java SDKs. If it cannot find one, it will prompt you to download one. (The Java Overview and this JDK Required page seems to also auto-appear when you open/create a .java file).

From the VS Code documentation, these JDK distributions should be supported: OpenJDK, Java SE from Oracle, Azul Zulu Enterprise. For this answer, I am using OpenJDK 11. Download and install the appropriate JDK.

Next, get the path to your JDK. Open a Terminal and then:

$ /usr/libexec/java_home
/Library/Java/JavaVirtualMachines/adoptopenjdk-11.jdk/Contents/Home

If you already have a JAVA_HOME env var set, I recommend not relying on it. I find that it is better to set the relevant java.* properties instead to configure Java specifically for VS Code (or for a particular workspace).

Now, open the VS Code Settings tab and look for the Java: Home setting:

You can modify either the User or Workspace setting. In the screenshot above, I am modifying my User settings, making the java.home/java.jdt.ls.java.home setting affect all Java projects. You can also modify the Workspace setting to configure java.home differently for each workspace.

Click on the Edit in settings.json, and add java.home (you'll know your extension was installed properly if java.* autocompletion is displayed).

Specify the path to the JDK you got earlier:

"java.home": "/Library/Java/JavaVirtualMachines/adoptopenjdk-11.jdk/Contents/Home",

Then restart VS Code. When the Java Overview tab displays again, if you set the path correctly, it should not prompt you anymore to download a JDK, meaning it found your JDK. Try running a very basic HelloWorld.java application.

Windows

If you are on Windows, they provide a special installer to help you get started:

To help you get started quickly, we created a special Installer of Visual Studio Code for Java developers.

Download Visual Studio Code Java Pack Installer

Note: The installer is currently only available for Windows. For other OS, please install those components (JDK, VS Code and Java extensions) individually. We're working on the macOS version, please stay tuned.

Only for 32bit START

+++Please mind that 32bit is usually outdated. This is probably just for those who have an old netbook lying around.+++

You can just switch to OpenJDK 14, which is recommended by vscode "itself" (Oracle JDK has stopped support for 32bit after version 8).

This vscode site https://github.com/redhat-developer/vscode-java/wiki/JDK-Requirements#jdk11.requirement recommends the following link right at the top at the Java Development Kit link:

https://adoptopenjdk.net/

A non-Oracle JDK is thus the only way to use java 14 on a 32bit.

Only for 32bit END

####

32bit and 64bit START

For those who cannot install JDK 11 or higher due to a 32bit system together with a not allowed OpenJDK JDK (that is, if Oracle JDK is obligatory), or those who have other restrictions on 64bit, one workaround is to install vscode-java version 0.64.1 instead of the most recent 0.65.0. (This again, is not recommended unless you have restrictions on the JDK version, it is just possible, see above)

See https://github.com/redhat-developer/vscode-java/issues/1543#issuecomment-669895753

How to install the earlier extension version:

EDIT

You should not do the following if you have no other restrictions. The most plausible restriction is that you use Windows 32bit and you have to use an Oracle JDK (Oracle JDK 8 is the last version supporting 32bit). Or if you must use Gradle with a version below 4.7, see the github discussion again.

In the usual case: upgrade, and then you can still go back to using an older java version by changing the settings according to https://github.com/redhat-developer/vscode-java/wiki/JDK-Requirements#jdk11.requirement:

Do I need to migrate my projects to Java 11?

"NO, you don't! Well, you should, be we're not here to judge. It is still possible to compile/run Java applications from Java 1.5 to 14, provided the proper java.configuration.runtimes are configured in the user's settings.json."

--> This means you can upgrade the extension and use the Java runtime of your choice. See detailed instructions for Windows 64bit at https://github.com/redhat-developer/vscode-java/issues/1543#issuecomment-671106893.

To config to the right JDK in VS Code. Open File -->Settings --> Java --> Configuration:Runtimes -->[Edit in Setting.json] add:

"java.configuration.runtimes": [
    {
        "name": "JavaSE-18",
        "path": "/usr/lib/jvm/java-18-openjdk-18.0.1.1.2-1.rolling.fc36.x86_64",
        "sources" : "/usr/lib/jvm/java-18-openjdk-18.0.1.1.2-1.rolling.fc36.x86_64/lib/src.zip",
        "javadoc" : "/usr/share/javadoc/java-18-openjdk-18.0.1.1.2-1.rolling.fc36.x86_64/api",
        "default":  true
    }
]

The path is the "$JAVA_HOME" if you run $sudo alternatives --config java and the result is

+ 3 java-latest-openjdk.x86_64 (/usr/lib/jvm/java-18-openjdk-18.0.1.1.2-1.rolling.fc36.x86_64/bin/java)

then JAVA_HOME is /usr/lib/jvm/java-18-openjdk-18.0.1.1.2-1.rolling.fc36.x86_64

The source, if you install sudo dnf install java-latest-openjdk-src.x86_64 the source code will be $JAVA_HOME/lib/src.zip

To find the Javadoc if you install "java-latest-openjdk-javadoc.x86_64", you can use command rpm -pl java-latest-openjdk-javadoc.x86_64 to check the installation location.

On Mac and VSCode Version: 1.86.2 I was unable to select an alternative JDK for my Java project. In particular I wanted to run my Java classes with a specific JDK.

I tried to add the JDK to the settings.json file ( with both a "java.jdt.ls.java.home" entry and a entry with "java.configuration.runtimes" but VSCode always run the wrong JDK. Even if setting proper JAVA_HOME before starting VSCode did not help.

Eventually I modified the launch.json file by adding a javaexec entry and then I could control what JDK to use! Below I pinpoint my JDK 21 installed using homebrew;

{
    "version": "0.2.0",
    "configurations": [
    {
        "type": "java",
        "name": "MyJavaClass",
        "javaExec": "/usr/local/Cellar/openjdk/21.0.1/libexec/openjdk.jdk/Contents/Home/bin/java",
        "request": "launch",
        "mainClass": "se.myorg.MyJavaClass",
        "projectName": "myproject"
    }]
}