First, find out what JDK version that is installed on your computer. To do this, run the following command in the terminal.
/usr/libexec/java_home -V
If you have any JDK installed, the output will be something simular to this.
Matching Java Virtual Machines (3):
11.0.1, x86_64: "Java SE 11.0.1" /Library/Java/JavaVirtualMachines/jdk-11.0.1.jdk/Contents/Home
1.8.0_202, x86_64: "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_202.jdk/Contents/Home
1.8.0_51, x86_64: "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_51.jdk/Contents/Home
Then, if your want to use the Java 1.8, then you should execute.
export JAVA_HOME=`/usr/libexec/java_home -v 1.8.0_202`
Then you can check if the changes have been taking into effect by checking the java version:
java -version