为什么同一段（简单的）Java代码在不同的Android设备上的行为会大不相同？

Question

Why same piece of (simple) Java code behaves very differently on different Android devices?

That simple piece of code is just the use of String.replace(CharSequence target, CharSequence replacement) with target == "" :

package com.example.stringreplacetest;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        String str = "just_a_string";

        System.out.println(str.replace("", "-"));
        ((TextView) findViewById(R.id.textView)).setText(str.replace("", "-"));
    }
}

It produces -just-_-a-_-string- on my LG Optimus 3D P920 (Android 2.3.3), and my sister's Samsung Galaxy S2 (Android 4.1.2) , and i guess on most of your devices as well.

But it halts (a suspect of infinite looping) on my LG Optimus Chic (Android 2.2).

The old LG Optimus Chic and Android 2.2 may be buggy. ( String.replace() indeed has a bug .) But the piece of code in String.replace() is relatively simple - "simple" means no dynamic binding, no Threads, etc...

Isn't that piece of code should be finalised during compile time? How does Java compiler work (as i know Java is a cross-platform language, it may work differently)?

PS to ensure it is the same piece of compiled code, i actually transferred the compiled .apk by USB to my Android phones, rather than using Eclipse to RUN them directly in the devices.

---

i have found the source code of Android 2.2 Froyo:

https://android.googlesource.com/platform/dalvik/+/froyo-release/libcore/luni/src/main/java/java/lang/String.java

It does cause infinite looping when target.length == 0 (because in the do-while loop , string.indexOf("", tail) will never return -1 ).

Doubts has been cleared a bit. But...

i still don't know why different versions of String class is loaded, upon running in different devices. Is it this is what meant to be cross-platform ?

Answer 1

Let me close this question.

i could not find concrete documentary references. But after my repeated trials, and a few researches of the "differences between Java vs C compiler". Yes, this is the Java behaviour - compile once, run everywhere (& debug everywhere).

This is why we need the Java VM. This is why Java compiles faster than C/C++. This is why Java runs slower than C/C++.

(i guess) While Java compiles, it only records the class signatures. Upon running on VM, it matches those signatures with the real implementations of the corresponding classes, compile them into machine codes just-in-time. That is why Java can compile once, run on different machines , because different VMs have their own implementations of the classes. This also lead to the problem that, if there is a bug in a certain version, the programmer can do nothing about it. It is because the actual buggy implementation is at the client side where the end-user is running the program. We need to wait the user to update their own VM.

PS in fact we can re-invent the wheel, re-write our own classes. So those classes will be attached into the deployed program.