Java Builder pattern with Generic type bounds
- by I82Much
Hi all,
I'm attempting to create a class with many parameters, using a Builder pattern rather than telescoping constructors. I'm doing this in the way described by Joshua Bloch's Effective Java, having private constructor on the enclosing class, and a public static Builder class. The Builder class ensures the object is in a consistent state before calling build(), at which point it delegates the construction of the enclosing object to the private constructor. Thus
public class Foo {
// Many variables
private Foo(Builder b) {
// Use all of b's variables to initialize self
}
public static final class Builder {
public Builder(/* required variables */) {
}
public Builder var1(Var var) {
// set it
return this;
}
public Foo build() {
return new Foo(this);
}
}
}
I then want to add type bounds to some of the variables, and thus need to parametrize the class definition. I want the bounds of the Foo class to be the same as that of the Builder class.
public class Foo<Q extends Quantity> {
private final Unit<Q> units;
// Many variables
private Foo(Builder<Q> b) {
// Use all of b's variables to initialize self
}
public static final class Builder<Q extends Quantity> {
private Unit<Q> units;
public Builder(/* required variables */) {
}
public Builder units(Unit<Q> units) {
this.units = units;
return this;
}
public Foo build() {
return new Foo<Q>(this);
}
}
}
This compiles fine, but the compiler is allowing me to do things I feel should be compiler errors. E.g.
public static final Foo.Builder<Acceleration> x_Body_AccelField =
new Foo.Builder<Acceleration>()
.units(SI.METER)
.build();
Here the units argument is not Unit<Acceleration> but Unit<Length>, but it is still accepted by the compiler.
What am I doing wrong here? I want to ensure at compile time that the unit types match up correctly.
