Fix field initialization and refactor DecompileClass

[leslieyip02]

Fix field initialization

The DecompileClass.decompileField always assigns an initializer:

jade/src/main/kotlin/org/ucombinator/jade/decompile/DecompileClass.kt

Lines 161 to 177 in 2dc48dd

	private fun decompileField(node: FieldNode): FieldDeclaration {
	// attrs (ignore?)
	val modifiers = Flag.toModifiers(Flag.fieldFlags(node.access))
	val annotations: NodeList<AnnotationExpr> = decompileAnnotations(
	node.visibleAnnotations,
	node.invisibleAnnotations,
	node.visibleTypeAnnotations,
	node.invisibleTypeAnnotations,
	)
	val type =
	if (node.signature != null) Signature.typeSignature(node.signature) else Descriptor.fieldDescriptor(node.desc)
	val name = SimpleName(node.name)
	val initializer = decompileLiteral(node.value)
	val variables = NodeList<VariableDeclarator>(VariableDeclarator(type, name, initializer))
	return FieldDeclaration(modifiers, annotations, variables)
	}

It does so by decompiling the node's value as a literal and using that literal as the field's initializer. However, consider the following:

// Foo.java
class Foo {
  public int value;
}

Under the current logic, node.value is null, so decompileLiteral(node.value) is also null. When running jade decompile Foo.class ., the result is

/** Source File: Foo.java
* Class-file Format Version: 63
* Source Debug Extension: null // See JSR-45 https://www.jcp.org/en/jsr/detail?id=045
*/
class Foo extends java.lang.Object {

    public int value = null;

    Foo() {
        super();
        return;
    }
}

This is wrong since null cannot be assigned to primitives. This can be fixed by omitting the initializer argument if node.value is null. Applying the fix, the decompiled result can then be recompiled:

/**
 * Source File: Foo.java
 * Class-file Format Version: 63
 * Source Debug Extension: null // See JSR-45 https://www.jcp.org/en/jsr/detail?id=045
 */
class Foo extends java.lang.Object {

    public int value;

    Foo() {
        super();
        return;
    }
}

If we do set an initial value, the value is set in the constructor (this seems correct?):

Before	After
class Foo { public int value = 42; }	class Foo extends java.lang.Object { public int value; Foo() { super(); this.value = 42; return; } }

Refactor

Additionally, DecompileClass was also refactored and improved:

1. Add KDoc for DecompileClass
2. Refactor DecompileClass
   a. De-bloat large methods (i.e. decompileMethod and decompileClass) with helper functions
   b. Readability improvements (i.e. renamed some variables for better clarity, reorganized certain sections of code)
   c. Adding file-level comment using JavadocComment instead of BlockComment (the previous approach had alignment issues)