How to pass parameters into Java method using QAndroidJniObject

Question

In my Qt app, if I have a method written in Java inside my MainActivity class like below,

public Boolean myJavaTestMethod() {
    return true;
}

I know that I can invoke the method in the following way:

QAndroidJniObject method_retval = QtAndroid::androidActivity().callObjectMethod<jboolean>("myJavaTestMethod");

Question:
Above is great and it works. But, how do I pass a string from Qt C++ side into myJavaTestMethod?

Lets say I want to call the below method which takes an input parameter String into it

public Boolean myJavaTestMethodWithParam(String str) {
    return true;
}

Environment:
I am using Qt 5.15.1 commercial version.

Eddie Lopez · Accepted Answer · 2021-12-06 21:55:58Z

1

You have to take the following steps:

Call your Java object's function using name and signature:

QAndroidJniObject result = activity.callObjectMethod("myJavaTestMethodWithParam", "(Ljava/lang/String;)B", myJString);

Where activity is just what QtAndroid::androidActivity() returns, myJavaTestMethodWithParam your activity function and "(Ljava/lang/String;)B" is its signature.

Turn your QString into a jstring:

QString helloString("Hello");

QAndroidJniObject string = QAndroidJniObject::fromString(helloString);
jstring myJString = string.object<jstring>();

To understand the signature part, check: https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/types.html#wp16432

More on calling Android/Java methods here: https://doc.qt.io/qt-5/qandroidjniobject.html#callObjectMethod-1

edited Dec 6, 2021 at 21:55

answered Dec 6, 2021 at 20:47

Eddie Lopez

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4 Comments

TheWaterProgrammer Over a year ago

Can you please add the code for how you got the activity object?

Eddie Lopez Over a year ago

Oh, that's just as you did, QtAndroid::androidActivity(), I just left it out to save space.

TheWaterProgrammer Over a year ago

This doesn't work. I think this answer is wrong. How can activity.myJavaTestMethodWithParam compile?

Eddie Lopez Over a year ago

I just saw what you said, I fixed it, myJavaTestMethodWithParam won't compile obviously, that's the method you want to call, it goes as the first argument of callObjectMethod(...). A mistake typing in the solution.

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