Java 类之间传递空字符串的问题

Question

I'm trying to pass a string from one class to another, but instead of it passing the string I need, it's passing a null value, which causes an error in the decryption function I'm trying to do. Here are the two classes with details on each one that I'm trying to achieve:

MainActivity -> trying to take the value I get from "String decrypted1 = d1.encryppass(encrypted);"

                    Decrypt d1 = new Decrypt();
                try {
                    if (cursor.moveToFirst()) {
                        do {
                            String name = cursor.getString(0);
                            String encrypted = cursor.getString(1);
                            d1.myMethod(this);
                            decrypted1 = d1.encryppass(encrypted);
                            Log.i("Title:     ", name);
                            Log.i("Password:  ", decrypted1);
                            Toast.makeText(getApplicationContext(), decrypted1, Toast.LENGTH_LONG).show();
                        } while (cursor.moveToNext());
                    }
                } catch (NullPointerException exception) {
                    exception.printStackTrace();
                }
            }

Decryption 2 class -> and pass that above statement here "private String pinToMd5 = d3.decrypted1;"

public class Decrypt2 {
    private Cipher cipher;
    MainActivity d3;
    private String pinToMd5;
    public String pin_to_md5;SecretKeySpec secretKeySpe = new SecretKeySpec(this.pin_to_md5.getBytes(), "AES");

    public Decrypt2(MainActivity mainActivity)
        {
        d3 = mainActivity;
        pinToMd5 = d3.decrypted1;
        pin_to_md5 = MD5(pinToMd5);
        }

Answer 1

From what I can see the issue might be here

MainActivity d3 = new MainActivity();
    //private String pinToMd5 = "PaSsW0rD"; //use this to test my code to confirm any changes doesn't break the logic -> this works
    private String pinToMd5 = d3.decrypted1;

You are creating new instance from MainActivity class and expect the object from the one presented in the stack. Try passing the context or reference from the existing main activity class and assign it in the Decrypt class

    InputStream instream = getContentResolver().openInputStream(singleUri);
/*Reference from MainActivity class*/
                        Decrypt2 d2 = new Decrypt2(this);
                        String decrypted2 = d2.encrypnote(instream);

public class Decrypt2 {
//your code...

MainActivity d3;
    //private String pinToMd5 = "PaSsW0rD"; //use this to test my code to confirm any changes doesn't break the logic -> this works
private String pinToMd5;

public Decrypt2(MainActivity mainActivity)
{
    d3 = mainActivity;
    pinToMd5 = d3.decrypted1;
}
//your code...
}

Not sure this will fix your problem completely :)

Second issue is this code here:

public String pin_to_md5 = MD5(pinToMd5);

You are invoking the method MD5 before the pinToMd5 property is initialised? when did you need to use that class? If on the initialization of the class than do this changes in the code:

public class Decrypt2 {
//your code...

MainActivity d3;
    //private String pinToMd5 = "PaSsW0rD"; //use this to test my code to confirm any changes doesn't break the logic -> this works
private String pinToMd5;
//remove MD5(pinToMd5);
public String pin_to_md5;
SecretKeySpec secretKeySpe;


public Decrypt2(MainActivity mainActivity)
{
    d3 = mainActivity;
    pinToMd5 = d3.decrypted1;
    pin_to_md5 = MD5(pinToMd5);
    secretKeySpe = new SecretKeySpec(this.pin_to_md5.getBytes(), "AES");

}
//your code...
}

Third issue In MainActivity you are using

public String decrypted1 = "";

as property. But then in onClick method you are creating local variable that contains the encrypted string

     d1.myMethod(this);
//this code here
     String decrypted1 = d1.encryppass(encrypted);
    
     Log.i("Title:     ", name);

Local values live only inside the method but not in the class so instead creating new variable in the method use the one from above. Remove that String .. and make the code smth like this

d1.myMethod(this);
//this code here
decrypted1 = d1.encryppass(encrypted);
        
Log.i("Title:     ", name);