Java：为什么我不能在一个语句中声明引用变量并在类的另一个语句中创建引用的对象？

Question

// That doesn't work: 

import java.io.File;

public class Test {
    File file1;
    file1 = new File("path");
}

//--------------------------------------

// The following works:

import java.io.File;

public class Test {
    File file1 = new File("path");
}

I don't understand why the first version is not possible. I also tried it with an int-value (which is not an object - I think):

//Also doesn't work:

public class Test {
    int number;
    number = 4;
} 

---

Thank you! I tried it and it works (without implementing a non-default constructor or a method):

import java.io.File;

public class Test {
    int number;
    {
        number = 4;
    }
    File file1;
    {
        file1 = new File("path");
    }
    public static void main(String[] args) {
        Test test = new Test();
        System.out.print(test.number + " , " + test.file1.getName());
// Output: 4 , path
    }
}

Answer 1

It's because you cannot have executable code in the class definition outside a method. So the line

file1 = new File("path");

(which is a statement), is illegal. It never gets executed. The class definition is processed at compile time, but the compiler is not a virtual machine, it doesn't execute your code. Statements are executed at runtime.

You can, as BM noted, create a static piece of code which is executed when the class is loaded. But, I believe it is equivalent to your second example:

File file1 = new File("path");

(but I admit not to have checked the bytecode for that).

Answer 2

You can do it with a block statement :

public class Test {
    File file1 ;
     {
        file1 = new File("path");
     }
}