Navigate to folder containing Java (JRE) installation. In the /bin you will find  keytool.exe which will aid you import the certificate to the Java keystore.

C:\Path to\Java\current JRE\bin>keytool -importcert -file nameofcertificate.cer -keystore "\Path to Java installation\lib\security\cacerts" -alias any unique name

C:\Program Files\Java\jre1.8.0_192\bin>keytool -importcert -file examplecertificate.cer -keystore "C:\Program Files\Java\jre1.8.0_192\lib\security\cacerts" -alias Example

You will be prompted to enter a password, unless you had originally changed the default password it will be changeit

The certificate will be imported and the you will be prompted with a question;

Trust this certificate? [no] Type in Yes

The final message will be;

Certificate was added to keystore