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JavaParser: How to retrieve all ParentNode names from MethodDeclaration?

I am using JavaParser library (https://github.com/javaparser/javaparser) for parsing Java method declarations. I want to identify different method declarations from different packages, classes, scopes, etc. so that I can precisely identify every method declarations.
For example:
Here is a Java file called MainClass.java:
package com.company.packA.packB;
public class MainClass {
void doTask(int x, int y) {
//...
}
private class InnerClass {
void doTask(int x, int y) {
//...
}
}
}
class AnotherClassSameFile {
void doTask(int x, int y) {
//...
}
}
Notice that the above example contains three void doTask(int x, int y) methods:
com.company.packA.packB→MainClass→doTask(int x, int y)
com.company.packA.packB→MainClass→InnerClass→doTask(int x, int y)
com.company.packA.packB→AnotherClassSameFile→doTask(int x, int y)
To identify different method declarations with the same method signatures, I need to traverse all parent nodes until the root node.
So far I have tried this code (simplified) using JavaParser library:
class MethodStruct { // the second example will be:
String parentNodes; // com.company.packA.packB#MainClass#InnerClass
String returnType; // void
String methodName; // doTask
String parameters; // int,int
}
class JavaParserTest {
// this is the method to be called from outside
static List<MethodStruct> getMethodStructs(Reader reader) {
CompilationUnit cu = JavaParser.parse(reader);
List<MethodStruct> methodStructs = new LinkedList<>();
cu.accept(new MethodVisitor(), methodStructs);
return methodStructs;
}
static class MethodVisitor extends VoidVisitorAdapter<List<MethodStruct>> {
#Override
public void visit(MethodDeclaration methodDeclaration, List<MethodStruct> methodStructs) {
super.visit(methodDeclaration, methodStructs);
// adding individual methodStruct into the list
methodStructs.add(getMethodStruct(methodDeclaration));
}
static MethodStruct getMethodStruct(MethodDeclaration methodDeclaration) {
return new MethodStruct(
getParents(methodDeclaration),
methodDeclaration.getTypeAsString(),
methodDeclaration.getNameAsString(),
getParameterAsString(methodDeclaration.getParameters()));
}
// it is the method to be concerned for my question
static String getParents(MethodDeclaration methodDeclaration) {
StringBuilder parents = new StringBuilder();
Node currentNode = methodDeclaration;
while (currentNode.getParentNode().isPresent()) {
// goto parent node
currentNode = currentNode.getParentNode().get();
//TODO: I'm stuck here. Help me please!
//TODO: How to identify each node whether
// it is a package, innerClass, etc?
}
// convert StringBuilder into String and return the String
return parents.toString();
}
static String getParameterAsString(NodeList<Parameter> parameters) {
// easy task! convert parameter string list
// into a single string (comma separated)
}
}
}
I am facing difficulties in defining my getParents(MethodDeclaration methodDeclaration) method. How can I solve it (i.e., identify every parent nodes)? I can't find any useful method of Node class for my goal. I might have missed something in JavaParser library.

You should try to use walk method to find all scopes of concrete method declaration:
static String getParents(fina MethodDeclaration methodDeclaration) {
final StringBuilder parents = new StringBuilder();
methodDeclaration.walk(Node.TreeTraversal.PARENTS, node -> {
if (node instanceof ClassOrInterfaceDeclaration) {
path.insert(0, ((ClassOrInterfaceDeclaration) node).getNameAsString());
path.insert(0, '$');
}
if (node instanceof ObjectCreationExpr) {
path.insert(0, ((ObjectCreationExpr) node).getType().getNameAsString());
path.insert(0, '$');
}
if (node instanceof MethodDeclaration) {
path.insert(0, ((MethodDeclaration) node).getNameAsString());
path.insert(0, '#');
}
if (node instanceof CompilationUnit) {
final Optional<PackageDeclaration> pkg = ((CompilationUnit) node).getPackageDeclaration();
if (pkg.isPresent()) {
path.replace(0, 1, ".");
path.insert(0, pkg.get().getNameAsString());
}
}
});
// convert StringBuilder into String and return the String
return parents.toString();
}

Applying Factory Pattern Extensively

I'm developing a game and there are tons of occasions where I need some sort of factory pattern involved.
In an attempt not to create lots of Factory class methods, I used Supplier<T> instead. This works great but not if there are required arguments.
It works in this case: () -> new Spawn(3, 6, "example");
But sometimes I need to pass other parameters to the factory.
There's the Consumer and BiConsumer which accept two parameters. But there's no interface for 3, 4, 5...
I came up with an embarassing solution to this problem, but it illustrates what I'm trying to achieve. What other solutions are there?
import java.util.function.Function;
public class FactoryExample {
static class Args {
Object[] objs;
Args(Object ...objs) { this.objs = objs; }
Object[] get() { return objs; }
}
static class Thing {
int a; char b; boolean c;
Thing(int a, char b, boolean c) {
this.a = a; this.b = b; this.c = c; }
}
static class Number {
int x;
Number(int x) { this.x = x; }
}
public static void main(String[] args) {
Function<Args, Number> factoryA = arg -> {
int x = (int) arg.get()[0];
return new Number(x);
};
Function<Args, Thing> factoryB = arg -> {
int a = (int) arg.get()[0];
char b = (char) arg.get()[1];
boolean c = (boolean) arg.get()[2];
return new Thing(a, b, c);
};
factoryB.apply(new Args(3, 'a', true));
factoryA.apply(new Args(3));
}
}
Example: how do I avoid creating a bunch of these factories?
public class InfectionFactory {
private Integer damage;
private Integer delay;
private Integer hits;
private Integer spikes;
private Color color;
public InfectionFactory setColor(Color color) {
this.color = color;
return this;
}
public InfectionFactory setSpikes(int spikes) {
this.spikes = spikes;
return this;
}
public InfectionFactory setDamage(int damage) {
this.damage = damage;
return this;
}
public InfectionFactory setDelay(int delay) {
this.delay = delay;
return this;
}
public InfectionFactory setHits(int hits) {
this.hits = hits;
return this;
}
public Infection create(Game game, Living target) {
Infection infection = new Infection(game, target);
if (damage != null) infection.setDamage(damage);
if (color != null) infection.setColor(color);
if (delay != null) infection.setDelay(delay);
if (hits != null) infection.setHits(hits);
if (spikes != null) infection.setSpikes(spikes);
return infection;
}
}

You seem to have multiple requirements. First of all, to make the Supplier take all the arguments you want, you can do something like:
public class SpawnFactory implements Supplier<Spawn> {
//todo: put members for all the arguments you need, make them final so that you don't miss any in the constructor
public SpawnFactory( ... all the arguments you want ... ) {
}
public Spawn get() {
return new Spawn( ... all the arguments you want ...);
}
}
Since it implements Supplier, you can pass them directly instantiated the way you like. Instead of () -> new Spawn(3, 6, "example") just do new SpawnFactory(3, 6, "example");
The example of your InfectionFactory on the other hand is following the builder pattern (you might want to rename it InfectionBuilder). Nothing is wrong with this (apart from the fact that it seems a bit redundant since Infection is using the same pattern too)
You might want to make it take the Game and Living as constructor arguments instead, and then your create() will not need any arguments. Your last step would be to make the class implement Supplier<Infection> and add Infection get() which just calls create() (unless you want to rename create() to get()).

Tree traversal in Java with Generic classes

To be precise, I am trying to flatten a tree and I am stuck on trying to get the values of private attributes in a generic class using a generic function.
I have attached the classes to show how the tree is structured exactly. But it's looks something like this:
/|\
1 | 6
/|\
5 4 9
I am going to paste my attempt at the end. First, let me introduce the classes:
Triple simply stores three values of the same type.
public class Triple<V> {
private final V l, m, r;
public Triple(V l, V m, V r) {
this.l = l;
this.m = m;
this.r = r;
}
public V left() { return l; }
public V middle() { return m; }
public V right() { return r; }
}
Straightforward interface:
public interface Function<P, R> {
R apply(P p);
}
Now, for a tricky class. This one is simply a type that stores one of EitherOr of two types of value, but not both.
public class EitherOr<A,B> {
// Constructs a left-type EitherOr
public static <A> EitherOr left(A a) {
return new EitherOr(a, null);
}
// Constructs a right-type EitherOr
public static <B> EitherOr right(B b) {
return new EitherOr(null, b);
}
private final A a;
private final B b;
private EitherOr(A a, B b) {
this.a = a; this.b = b;
}
public<T> T ifLeft(Function<A,T> f) {
return f.apply(a);
}
public<T> T ifRight(Function<B,T> f) {
return f.apply(b);
}
public boolean isLeft() {
return b == null;
}
}
I know this is getting long, but bear with me. This class implements the tree structure.
public interface Tree<T> {
EitherOr<T, Triple<Tree<T>>> get();
static final class Leaf<T> implements Tree<T> {
public static <T> Leaf<T> leaf (T value) {
return new Leaf<T>(value);
}
private final T t;
public Leaf(T t) { this.t = t; }
#Override
public EitherOr<T, Triple<Tree<T>>> get() {
return EitherOr.left(t);
}
}
static final class Node<T> implements Tree<T> {
public static <T> Tree<T> tree (T left, T middle, T right) {
return new Node<T>(Leaf.leaf(left), Leaf.leaf(middle), Leaf.leaf(right));
}
private final Triple<Tree<T>> branches;
public Node(Tree<T> left, Tree<T> middle, Tree<T> right) {
this.branches = new Triple<Tree<T>>(left, middle, right);
}
#Override
public EitherOr<T, Triple<Tree<T>>> get() {
return EitherOr.right(branches);
}
}
}
Alright. Here is my idea for flattening:
public class MyFlattenTree<T> implements FlattenTree<T> {
public List<T> flattenInOrder(Tree<T> tree) {
List<T> list = new ArrayList<T>();
EitherOr<T, Triple<Tree<T>>> EitherOr;
EitherOr = tree.get();
// it is a leaf
if (EitherOr.isLeft()) {
// This is where the problem lies
// I don't how to get the value using a function f
list.add((T) EitherOr.ifLeft(f));
return list;
}
else {
// basically recursively go through the tree somehow
}
return null;
}
}
As I said, I am stuck with trying to retreive the value in the EitherOr class using the Function interface. I am thinking of implementing the Function interface and write a function for "apply" that just gets the value, but I am not sure how to do that. Any help would be appreciated. Thanks!

So, here is your flattenInOrder method:
public List<T> flattenInOrder(final Tree<T> tree) {
final EitherOr<T, Triple<Tree<T>>> EitherOr = tree.get();
if (EitherOr.isLeft()) {
return Collections.singletonList(EitherOr.ifLeft(this.ifLeftFunction));
}
return EitherOr.ifRight(this.ifRightFunction);
}
Quite simple, assuming that:
ifLeftFunction yields a single element (since EitherOr<T, Triple<Tree<T>>> has a single T elem' if it s "left")
... and:
ifRightFunction yields a collection of elements (since EitherOr<T, Triple<Tree<T>>> has a list of T elems' if it is "right")
Let's look into these functions now:
ifLeftFunction is... basic. I want to extract a T from... a T.
final Function<T, T> ifLeftFunction = new Function<T, T>() {
#Override
public T apply(final T t) {
return t;
}
};
ifRightFunction is slightly more complex: it has to be recursive and collect all Ts from the Tree it's browsing:
final Function<Triple<Tree<T>>, List<T>> ifRightFunction = new Function<Triple<Tree<T>>, List<T>>() {
#Override
public List<T> apply(final Triple<Tree<T>> t) {
final List<T> res = new ArrayList<>();
res.addAll(MyFlattenTree.this.flattenInOrder(t.left()));
res.addAll(MyFlattenTree.this.flattenInOrder(t.middle()));
res.addAll(MyFlattenTree.this.flattenInOrder(t.right()));
return res;
}
};
And... you're done!
Sample working code:
public class MyFlattenTree<T> {
private final Function<Triple<Tree<T>>, List<T>> ifRightFunction = new Function<Triple<Tree<T>>, List<T>>() {
#Override
public List<T> apply(final Triple<Tree<T>> t) {
final List<T> res = new ArrayList<>();
res.addAll(MyFlattenTree.this.flattenInOrder(t.left()));
res.addAll(MyFlattenTree.this.flattenInOrder(t.middle()));
res.addAll(MyFlattenTree.this.flattenInOrder(t.right()));
return res;
}
};
private final Function<T, T> ifLeftFunction = new Function<T, T>() {
#Override
public T apply(final T t) {
return t;
}
};
public static void main(final String[] args) {
final Tree<String> tree = new Node<>(new Leaf<>("1"), new Node<>(new Leaf<>("5"), new Leaf<>("4"), new Leaf<>("9")), new Leaf<>("6"));
System.out.println(new MyFlattenTree<String>().flattenInOrder(tree));
}
public List<T> flattenInOrder(final Tree<T> tree) {
final EitherOr<T, Triple<Tree<T>>> EitherOr = tree.get();
if (EitherOr.isLeft()) {
return Collections.singletonList(EitherOr.ifLeft(this.ifLeftFunction));
}
return EitherOr.ifRight(this.ifRightFunction);
}
}
Note that I'm creating the exact Tree you're featuring as an example in your question in the main method here:
public static void main(final String[] args) {
final Tree<String> tree = new Node<>(new Leaf<>("1"), new Node<>(new Leaf<>("5"), new Leaf<>("4"), new Leaf<>("9")), new Leaf<>("6"));
System.out.println(new MyFlattenTree<String>().flattenInOrder(tree));
}
Output: [1, 5, 4, 9, 6]
Cheers ;)

In java, can you use the builder pattern with required and reassignable fields?

This is related to the following question:
How to improve the builder pattern?
I'm curious whether it's possible to implement a builder with the following properties:
Some or all parameters are required
No method receives many parameters (i.e., no list of defaults supplied to the initial builder factory method)
All builder fields can be reassigned an arbitrary number of times
The compiler should check that all parameters have been set
It is ok to require that parameters are initially set in some order, but once any parameter is set, all following builders can have this parameter set again (i.e., you can reassign the value of any field of the builder you wish)
No duplicate code should exist for setters (e.g., no overriding setter methods in builder subtypes)
One failed attempt is below (empty private constructors omitted). Consider the following toy builder implementation, and note that line with "Foo f2" has a compiler error because the inherited setter for a returns a BuilderB, not a BuilderFinal. Is there a way to use the java type system to parameterize the return types of the setters to achieve the above goals, or achieve them some other way.
public final class Foo {
public final int a;
public final int b;
public final int c;
private Foo(
int a,
int b,
int c) {
this.a = a;
this.b = b;
this.c = c;
}
public static BuilderA newBuilder() {
return new BuilderC();
}
public static class BuilderA {
private volatile int a;
public BuilderB a(int v) {
a = v;
return (BuilderB) this;
}
public int a() {
return a;
}
}
public static class BuilderB extends BuilderA {
private volatile int b;
public BuilderC b(int v) {
b = v;
return (BuilderC) this;
}
public int b() {
return b;
}
}
public static class BuilderC extends BuilderB {
private volatile int c;
public BuilderFinal c(int v) {
c = v;
return (BuilderFinal) this;
}
public int c() {
return c;
}
}
public static class BuilderFinal extends BuilderC {
public Foo build() {
return new Foo(
a(),
b(),
c());
}
}
public static void main(String[] args) {
Foo f1 = newBuilder().a(1).b(2).c(3).build();
Foo f2 = newBuilder().a(1).b(2).c(3).a(4).build();
}
}

Your requirements are really hard, but see if this generic solution fits the bill:
public final class Foo {
public final int a;
public final int b;
public final int c;
private Foo(
int a,
int b,
int c) {
this.a = a;
this.b = b;
this.c = c;
}
public static BuilderA<? extends BuilderB<?>> newBuilder() {
return new BuilderFinal();
}
public static class BuilderA<T extends BuilderB<?>> {
private volatile int a;
#SuppressWarnings("unchecked")
public T a(int v) {
a = v;
return (T) this;
}
public int a() {
return a;
}
}
public static class BuilderB<T extends BuilderC<?>> extends BuilderA<T> {
private volatile int b;
#SuppressWarnings("unchecked")
public T b(int v) {
b = v;
return (T) this;
}
public int b() {
return b;
}
}
public static class BuilderC<T extends BuilderFinal> extends BuilderB<T> {
private volatile int c;
#SuppressWarnings("unchecked")
public T c(int v) {
c = v;
return (T) this;
}
public int c() {
return c;
}
}
public static class BuilderFinal extends BuilderC<BuilderFinal> {
public Foo build() {
return new Foo(
a(),
b(),
c());
}
}
public static void main(String[] args) {
Foo f1 = newBuilder().a(1).b(2).c(3).build();
Foo f2 = newBuilder().a(1).b(2).c(3).a(4).build();
}
}

To my knowledge the builder pattern should be used in case multiple parameters are used that make the invocation rather complicated as parameters might swap positions or not make it obviously clear what which parameter is for.
A rule of thumb would be to require compulsory parameters within the constructor of the builder and optional parameters within the methods. However, often more than 4 parameters may be required which makes the invocation again rather unclear and the pattern redundant. So a split up into default constructor and parameter setting for each parameter can also be used.
The checks should happen in a own method which is invoked within the build-method so you could invoke it using super. Compile-time security is only guaranteed on the correct data types (only exception - null is possible to, this has to be fetched within the checkParameters()-method). You can however force that all required parameters are set on requiring them within the Builder constructor - but as mentioned before, this may lead to a redundant pattern.
import java.util.ArrayList;
import java.util.List;
public class C
{
public static class Builder<T extends C, B extends C.Builder<? extends C,? extends B>> extends AbstractBuilder<C>
{
protected String comp1;
protected String comp2;
protected int comp3;
protected int comp4;
protected int comp5;
protected List<Object> comp6 = new ArrayList<>();
protected String optional1;
protected List<Object> optional2 = new ArrayList<>();
public Builder()
{
}
public B withComp1(String comp1)
{
this.comp1 = comp1;
return (B)this;
}
public B withComp2(String comp2)
{
this.comp2 = comp2;
return (B)this;
}
public B withComp3(int comp3)
{
this.comp3 = comp3;
return (B)this;
}
public B withComp4(int comp4)
{
this.comp4 = comp4;
return (B)this;
}
public B withComp5(int comp5)
{
this.comp5 = comp5;
return (B)this;
}
public B withComp6(Object comp6)
{
this.comp6.add(comp6);
return (B)this;
}
public B withOptional1(String optional1)
{
this.optional1 = optional1;
return (B)this;
}
public B withOptional2(Object optional2)
{
this.optional2.add(optional2);
return (B)this;
}
#Override
protected void checkParameters() throws BuildException
{
if (this.comp1 == null)
throw new BuildException("Comp1 violates the rules");
if (this.comp2 == null)
throw new BuildException("Comp2 violates the rules");
if (this.comp3 == 0)
throw new BuildException("Comp3 violates the rules");
if (this.comp4 == 0)
throw new BuildException("Comp4 violates the rules");
if (this.comp5 == 0)
throw new BuildException("Comp5 violates the rules");
if (this.comp6 == null)
throw new BuildException("Comp6 violates the rules");
}
#Override
public T build() throws BuildException
{
this.checkParameters();
C c = new C(this.comp1, this.comp2,this.comp3, this.comp4, this.comp5, this.comp6);
c.setOptional1(this.optional1);
c.setOptional2(this.optional2);
return (T)c;
}
}
private final String comp1;
private final String comp2;
private final int comp3;
private final int comp4;
private final int comp5;
private final List<?> comp6;
private String optional1;
private List<?> optional2;
protected C(String comp1, String comp2, int comp3, int comp4, int comp5, List<?> comp6)
{
this.comp1 = comp1;
this.comp2 = comp2;
this.comp3 = comp3;
this.comp4 = comp4;
this.comp5 = comp5;
this.comp6 = comp6;
}
public void setOptional1(String optional1)
{
this.optional1 = optional1;
}
public void setOptional2(List<?> optional2)
{
this.optional2 = optional2;
}
// further methods omitted
#Override
public String toString()
{
StringBuilder sb = new StringBuilder();
sb.append(this.comp1);
sb.append(", ");
sb.append(this.comp2);
sb.append(", ");
sb.append(this.comp3);
sb.append(", ");
sb.append(this.comp4);
sb.append(", ");
sb.append(this.comp5);
sb.append(", ");
sb.append(this.comp6);
return sb.toString();
}
}
On extending D from C and also the builder, you need to override the checkParameters() and build() method. Due to the use of Generics the correct type will be return on invoking build()
import java.util.List;
public class D extends C
{
public static class Builder<T extends D, B extends D.Builder<? extends D, ? extends B>> extends C.Builder<D, Builder<D, B>>
{
protected String comp7;
public Builder()
{
}
public B withComp7(String comp7)
{
this.comp7 = comp7;
return (B)this;
}
#Override
public void checkParameters() throws BuildException
{
super.checkParameters();
if (comp7 == null)
throw new BuildException("Comp7 violates the rules");
}
#Override
public T build() throws BuildException
{
this.checkParameters();
D d = new D(this.comp1, this.comp2, this.comp3, this.comp4, this.comp5, this.comp6, this.comp7);
if (this.optional1 != null)
d.setOptional1(optional1);
if (this.optional2 != null)
d.setOptional2(optional2);
return (T)d;
}
}
protected String comp7;
protected D(String comp1, String comp2, int comp3, int comp4, int comp5, List<?> comp6, String comp7)
{
super(comp1, comp2, comp3, comp4, comp5, comp6);
this.comp7 = comp7;
}
#Override
public String toString()
{
StringBuilder sb = new StringBuilder();
sb.append(super.toString());
sb.append(", ");
sb.append(this.comp7);
return sb.toString();
}
}
The abstract builder class is quite simple:
public abstract class AbstractBuilder<T>
{
protected abstract void checkParameters() throws BuildException;
public abstract <T> T build() throws BuildException;
}
The exception is simple too:
public class BuildException extends Exception
{
public BuildException(String msg)
{
super(msg);
}
}
And last but not least the main method:
public static void main(String ... args)
{
try
{
C c = new C.Builder<>().withComp1("a1").withComp2("a2").withComp3(1)
.withComp4(4).withComp5(5).withComp6("lala").build();
System.out.println("c: " + c);
D d = new D.Builder<>().withComp1("d1").withComp2("d2").withComp3(3)
.withComp4(4).withComp5(5).withComp6("lala").withComp7("d7").build();
System.out.println("d: " + d);
C c2 = new C.Builder<>().withComp1("a1").withComp3(1)
.withComp4(4).withComp5(5).withComp6("lala").build();
System.out.println(c2);
}
catch (Exception e)
{
e.printStackTrace();
}
}
Output:
c: a1, a2, 1, 4, 5, [lala]
d: d1, d2, 3, 4, 5, [lala], d7
Builders.BuildException: Comp2 violates the rules
... // StackTrace omitted
Though, before messing to much with Generics I'd suggest to stick to the KISS policy and forget inheritance for builders and code them simple and stupid (with part of them including dumb copy&paste)
#edit: OK, after all the work done and re-reading the OP as well as the linked post I had a totally wrong assumption of the requirements - like a German wording says: "Operation successful, patient is dead" - though I leave this post here in case someone wants a copy&paste like solution for a builder-inheritance which actually returns the correct type instead of the the base type

I had a crazy idea once, and it kind of goes against some of your requirements, but I think you can have the builder constructor take the required parameters, but in a way that makes it still clear which parameters are being set. Take a look:
package myapp;
public final class Foo {
public final int a;
public final int b;
public final int c;
private Foo(int a, int b, int c) {
this.a = a;
this.b = b;
this.c = c;
}
public static class Builder {
private int a;
private int b;
private int c;
public Builder(A a, B b, C c) {
this.a = a.v;
this.b = b.v;
this.c = c.v;
}
public Builder a(int v) { a = v; return this; }
public Builder b(int v) { b = v; return this; }
public Builder c(int v) { c = v; return this; }
public Foo build() {
return new Foo(a, b, c);
}
}
private static class V {
int v;
V(int v) { this.v = v; }
}
public static class A extends V { A(int v) { super(v); } }
public static class B extends V { B(int v) { super(v); } }
public static class C extends V { C(int v) { super(v); } }
public static A a(int v) { return new A(v); }
public static B b(int v) { return new B(v); }
public static C c(int v) { return new C(v); }
public static void main(String[] args) {
Foo f1 = new Builder(a(1), b(2), c(3)).build();
Foo f2 = new Builder(a(1), b(2), c(3)).a(4).build();
}
}
For other clients, static imports are your friends:
package myotherapp;
import myapp.Foo;
import static myapp.Foo.*;
public class Program {
public static void main(String[] args) {
Foo f1 = new Builder(a(1), b(2), c(3)).build();
Foo f2 = new Builder(a(1), b(2), c(3)).a(4).build();
}
}

Building on Jordão's idea, I came up with the following, which may arguably satisfy all requirements 1-6 even though there is some duplicate code in the type parameters. Essentially, the idea is to "pass around" the return types of each method by using type parameters to override the return value of the inherited methods. Even though the code is verbose and impractical, and actually requires Omega(n^3) characters if you extend it out to an arbitrary number of fields n, I'm posting it because I think it's an interesting use of the java type system. If anyone can find a way to reduce the number of type parameters (especially asymptotically), please post in the comments or write another answer.
public final class Foo {
public final int a;
public final int b;
public final int c;
private Foo(
int a,
int b,
int c) {
this.a = a;
this.b = b;
this.c = c;
}
public static BuilderA<? extends BuilderB<?, ?>, ? extends BuilderC<?, ?>> newBuilder() {
return new BuilderFinal();
}
public static class BuilderA<B extends BuilderB<?, ?>, C extends BuilderC<?, ?>> {
private volatile int a;
#SuppressWarnings("unchecked")
public B a(int v) {
a = v;
return (B) this;
}
public int a() {
return a;
}
}
public static class BuilderB<B extends BuilderB<?, ?>, C extends BuilderC<?, ?>> extends BuilderA<B, C> {
private volatile int b;
#SuppressWarnings("unchecked")
public C b(int v) {
b = v;
return (C) this;
}
public int b() {
return b;
}
}
public static class BuilderC<B extends BuilderC<?, ?>, C extends BuilderC<?, ?>> extends BuilderB<B, C> {
private volatile int c;
#SuppressWarnings("unchecked")
public BuilderFinal c(int v) {
c = v;
return (BuilderFinal) this;
}
public int c() {
return c;
}
}
public static class BuilderFinal extends BuilderC<BuilderFinal, BuilderFinal> {
public Foo build() {
return new Foo(
a(),
b(),
c());
}
}
public static void main(String[] args) {
Foo f1 = newBuilder().a(1).b(2).c(3).a(2).build();
Foo f2 = newBuilder().a(1).a(2).c(3).build(); // compile error
Foo f3 = newBuilder().a(1).b(2).a(3).b(4).b(5).build(); // compile error
}
}

Why don't you want to override the setters in BuilderFinal? They would just need to downcast the super method:
public static class BuilderFinal extends BuilderC {
#Override
public BuilderFinal a(int v) {
return (BuilderFinal) super.a(v);
}
#Override
public BuilderFinal b(int v) {
return (BuilderFinal) super.b(v);
}
public Foo build() {
return new Foo(
a(),
b(),
c());
}
}

Implementing toString method in Java with The Visitor Pattern?

Here is some of my Java code
public List<OBJ> a = new ArrayList<OBJ>();
public String A;
public String B;
public String C;
for (OBJ o : a) {
// .... TODO
}
So I have an interface OBJ and there are three objects that implements OBJ say X, Y, Z. So I store X/Y/Z objects in List a. Now say that I want to go through the loop and if o is of instance X store X.value in A, if Y store Y.value in B, and if Z store Z.value in C. So the problem is really how do you figure out what object type o is (X,Y,Z) to store their values in the right string.
NOTE: I want to use the Visitor pattern or something like it, but I don't really have a firm grasp of it so I'm asking for your help.
This means NO Instanceof(s) or Type Casts and NO Dedicated Methods like
interface OBJ {
void blah();
}
class X implements OBJ {
public void blah();
} // etc
Thanks! I really want to get this import aspect of software engineering down!
Hey wow thanks for the detailed and fast responses, but my situation is a bit more complicated and sorry I didn't add this before.
So String A, B, C are actually housed in another class like
class ARGH {
public List<OBJ> a = new ArrayList<OBJ>();
public String A;
public String B;
public String C;
//invisible constructor here
public String toString () {
for (OBJ o : a) {
// .... TODO
}
return "some string"
}
}
public void main (String[] args) {
ARGH argh = new ARGH();
// Setup some X, Y, Z objects and their values here
String D = argh.toString();
// Do something to D
}
So the Strings and List are actually not global variables so I don't think this would work:
ObjVisitor v = new ObjVisitor() {
#Override
public void visit(X x) {
A = x.value();
}
// And so on.
}
I am assuming I have to somehow pass in the String A, B, C into the visit method but I don't know how to do that and still stay with The Visitor Pattern.

In a nut-shell you'd do like this:
Create a Visitor interface ObjVisitor with one visit-method for each type.
Add an abstract accept(ObjVisitor v) to OBJ.
Add an accept(ObjVisitor v) { v.visit(this); } to each OBJ implementation.
Call o.accept(yourVisitorImpl) in the loop.
You did indeed get some code from Bringer128. I elaborated a bit and added the String-stuff.
import java.util.*;
interface OBJ {
String accept(ObjVisitor v);
}
interface ObjVisitor {
String visit(X x);
String visit(Y y);
String visit(Z z);
}
class X implements OBJ {
public String accept(ObjVisitor v){ return v.visit(this); }
}
class Y implements OBJ {
public String accept(ObjVisitor v) { return v.visit(this); }
}
class Z implements OBJ {
public String accept(ObjVisitor v) { return v.visit(this); }
}
Usage:
class Test {
public static void main(String[] args) {
List<OBJ> objs = Arrays.asList(new Z(), new X());
ObjVisitor toStringVisitor = new ObjVisitor() {
public String visit(X x) { return "X object"; }
public String visit(Y y) { return "Y object"; }
public String visit(Z z) { return "Z object"; }
};
String result = "";
for (OBJ o : objs)
result += o.accept(toStringVisitor) + "\n";
System.out.println(result);
// Prints
// Z object
// X object
}
}
An alternative (perhaps better approach) would be to let the visitor implementation maintain a StringBuilder, let the visit-methods return void, and after the loop just call stringVisitor.getCurrentString() or something like that.

aioobe spelled it out for you, but here is what the implementation would look like.
interface OBJ {
void blah();
// New method!
void accept(ObjVisitor v);
}
class X implements OBJ {
public void blah() {}
#Override
public void accept(ObjVisitor v) {
v.visit(this);
}
}
interface ObjVisitor {
public void visit(X x);
public void visit(Y y);
}
Now use it:
public List<OBJ> a = new ArrayList<OBJ>();
public String A;
public String B;
public String C;
public void myMethod() {
ObjVisitor v = new ObjVisitor() {
#Override
public void visit(X x) {
A = x.value();
}
// And so on.
}
for (OBJ o : a) {
o.accept(v);
}
}