Android/Java 问题。 这两种决策树有何不同？

Question

I'm very confused as to how these two decision trees are different. I'm building an app that needs to decide which view to load based on the position selected from a ListView. I've tried to build the logic into a single controller module and found that the switch-case will cause a NullPointerException and FC while the if-else will work perfectly. Can anyone enlighten me as to why? I've got a strong background in C and C++ and am used to being able to easily re-write switches to if-else's and vice versa.

Defined vars:

private final int VALUEA = 0;
private final int VALUEB = 1;
private final int VALUEC = 2;

Switch-case:

TextView t = new TextView(null);
switch(value){
   case VALUEA:
        setContentView(R.layout.valuealayout);
        t = (TextView) findViewById(R.id.valuealayout);
        t.findViewById(R.id.valuealayout);
   break;
   case VALUEB:
        setContentView(R.layout.valueblayout);
        t = (TextView) findViewById(R.id.valueblayout);
        t.findViewById(R.id.valueblayout);
   break;
   case VALUEC:
        setContentView(R.layout.valueclayout);
        t = (TextView) findViewById(R.id.valueclayout);
        t.findViewById(R.id.valueclayout);
   break;
   default:
   break;
}

The block above will cause a NullPointerException.

If-else:

if(value == VALUEA ){
   setContentView(R.layout.valuealayout);
   TextView t = (TextView) findViewById(R.id.valuealayout);
   t.findViewById(R.id.valuealayout);
}else if(value == VALUEB){

   setContentView(R.layout.valueblayout);
   TextView t = (TextView) findViewById(R.id.valueblayout);
   t.findViewById(R.id.valueblayout);
}else if(value == VALUEC){
   setContentView(R.layout.valueclayout);
   TextView t = (TextView) findViewById(R.id.valueclayout);
   t.findViewById(R.id.valueclayout);
}else{
}

This version works perfectly. Does the second block work because of some funky Java scoping rule that allows each branch of the decision tree to create and properly initialize the TextView in a way that the first block doesn't?

Answer 1

The TextView constructor requires a Context . You can't just pass it null . Instead do:

TextView t = null;
switch(value){
   case VALUEA:
        setContentView(R.layout.valuealayout);
        t = (TextView) findViewById(R.id.valuealayout);
        t.findViewById(R.id.valuealayout);
        break;
   case VALUEB:
        setContentView(R.layout.valueblayout);
        t = (TextView) findViewById(R.id.valueblayout);
        t.findViewById(R.id.valueblayout);
        break;
   case VALUEC:
        setContentView(R.layout.valueclayout);
        t = (TextView) findViewById(R.id.valueclayout);
        t.findViewById(R.id.valueclayout);
        break;
   default:
        break;
}

Answer 2

I'm guessing it's the line

TextView t = new TextView(null);

that's the problem. Is it legal to pass null to the TextView constructor?

Without seeing the stack trace this is just a stab in the dark.