CodenameOne Java Hello World 示例中的 String.Format“错误：找不到符号”

Question

A super-simple String.format("this is a test %d",5) doesn't work in my HelloWorld CodenameOne project: I get "error: cannot find symbol".

It doesn't seem to matter what format I used, I always get the same error. This seems to be an import problem, though I'm not importing any special packages outside of the defaults.

Here is the java source:

package com.test.test;


import static com.codename1.ui.CN.*;
import com.codename1.ui.Display;
import com.codename1.ui.Form;
import com.codename1.ui.Dialog;
import com.codename1.ui.Label;
import com.codename1.ui.plaf.UIManager;
import com.codename1.ui.util.Resources;
import com.codename1.io.Log;
import com.codename1.ui.Toolbar;
import java.io.IOException;
import com.codename1.ui.layouts.BoxLayout;
import com.codename1.io.NetworkEvent;

/**
 * This file was generated by <a href="https://www.codenameone.com/">Codename One</a> for the purpose 
 * of building native mobile applications using Java.
 */
public class MyApplication {

    private Form current;
    private Resources theme;

    public void init(Object context) {
        // use two network threads instead of one
        updateNetworkThreadCount(2);

        theme = UIManager.initFirstTheme("/theme");

        // Enable Toolbar on all Forms by default
        Toolbar.setGlobalToolbar(true);

        // Pro only feature
        Log.bindCrashProtection(true);

/*
Updating property file: C:\Users\admin\Desktop\test2\build\built-jar.properties
Compile is forcing compliance to the supported API's/features for maximum device compatibility. This allows smaller
        code size and wider device support
Compiling 1 source file to C:\Users\admin\Desktop\test2\build\tmp
C:\Users\admin\Desktop\test2\src\com\test\test\MyApplication.java:39: error: cannot find symbol
        s = String.format("this is a test %d",5);
  symbol:   method format(String,int)
  location: class String
1 error
C:\Users\admin\Desktop\test2\build.xml:62: Compile failed; see the compiler error output for details.
BUILD FAILED (total time: 0 seconds)        
*/        
        String s;
        s = String.format("this is a test %d",5);

        addNetworkErrorListener(err -> {
            // prevent the event from propagating
            err.consume();
            if(err.getError() != null) {
                Log.e(err.getError());
            }
            Log.sendLogAsync();
            Dialog.show("Connection Error", "There was a networking error in the connection to " + err.getConnectionRequest().getUrl(), "OK", null);
        });        
    }
    
    public void start() {
        if(current != null){
            current.show();
            return;
        }
        Form hi = new Form("Hi World", BoxLayout.y());
        hi.add(new Label("Hi World"));
        hi.show();
    }

    public void stop() {
        current = getCurrentForm();
        if(current instanceof Dialog) {
            ((Dialog)current).dispose();
            current = getCurrentForm();
        }
    }
    
    public void destroy() {
    }

}

Answer 1

CodenameOne compiles your source code using its own subset of the Java SE API, which is missing some features that the standard Java API includes.

Quoting their FAQ :

What features of Java are supported? What features of Java aren't supported?

The most obvious thing missing is reflections. The main problem is that when we package the VM into devices that don't have Java, we would have to include EVERYTHING. If reflections were included, they wouldn't work anyway since we obfuscate the code for the platforms where reflections do work (eg Android). On top of that reflection code is generally slow and a bad idea on a mobile device to begin with. As an alternative some developers were successful with bytecode manipulation which is something that is completely seamless to the server and as performant/efficient as handcoding.

Many of the desktop API's such as java.net, java.io.File etc. aren't very appropriate for mobile devices and just didn't make it. We provide our own alternatives which are more portable and better suited for mobile settings.

Of the other missing things, if you run into a missing method or ability, there are cases where that functionality can be added.

Specifically, its version of java.lang.String does not include the format method.

In this case, it can be rewritten using simple string concatenation:

    String s = "this is a test " + 5;