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Q&A Trouble understanding java lambdas and generics

Of course, everyone thinks to understand lambdas and generics. I was one of those until recently. Look at my code: package debug; import java.util.function.Function; public class Understand...

1 answer · posted 11mo ago by watchmaker‭ · last activity 11mo ago by r~~‭

Question java

#2: Post edited by

ArtOfCode‭ · 2025-10-13T15:18:56Z (11 months ago)
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Of course, everyone *thinks* to understand lambdas and generics. I was one of those until recently.
Look at my code:
>
package debug;
import java.util.function.Function;
public class UnderstandingGenerix {
public static void main(String[] args) {
{
Function<?, ?> code = (String O) -> {
System.out.println("todo" + O);
return Double.valueOf(1.5);
};
// Object x = code.apply("xxx"); // compile error
// Object x = code.apply((Object) "xxx"); // compile error
// Double x = code.apply("xxx"); // compile error
}
{
Function<Object, Object> code = (O) -> {
System.out.println("todo" + O);
return Double.valueOf(1.5);
};
Object x = code.apply("xxx");
x = code.apply((Object) "xxx");
Double xx = (Double) code.apply("xxx");
}
{
Function code = (O) -> {
System.out.println("todo" + O);
return 1.5;
};
Object x = code.apply("xxx");
x = code.apply((Object) "xxx");
Double xx = (Double) code.apply("xxx");
}
}
}
~~(Ooops, why the dangling brace?)~~
From the net, the question mark is a "wildcard" matching *everything* (except primitive types). So "?" and "Object" should be equivalent.
What part did I miss?

Of course, everyone *thinks* to understand lambdas and generics. I was one of those until recently.
Look at my code:
```java
package debug;
import java.util.function.Function;
public class UnderstandingGenerix {
public static void main(String[] args) {
{
Function<?, ?> code = (String O) -> {
System.out.println("todo" + O);
return Double.valueOf(1.5);
};
// Object x = code.apply("xxx"); // compile error
// Object x = code.apply((Object) "xxx"); // compile error
// Double x = code.apply("xxx"); // compile error
}
{
Function<Object, Object> code = (O) -> {
System.out.println("todo" + O);
return Double.valueOf(1.5);
};
Object x = code.apply("xxx");
x = code.apply((Object) "xxx");
Double xx = (Double) code.apply("xxx");
}
{
Function code = (O) -> {
System.out.println("todo" + O);
return 1.5;
};
Object x = code.apply("xxx");
x = code.apply((Object) "xxx");
Double xx = (Double) code.apply("xxx");
}
}
}
```
From the net, the question mark is a "wildcard" matching *everything* (except primitive types). So "?" and "Object" should be equivalent.
What part did I miss?

#1: Initial revision by

watchmaker‭ · 2025-10-13T13:44:59Z (11 months ago)

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Trouble understanding java lambdas and generics

Of course, everyone *thinks* to understand lambdas and generics. I was one of those until recently.

Look at my code:

 > 
package debug;

import java.util.function.Function;

public class UnderstandingGenerix {

	public static void main(String[] args) {

		{
			Function<?, ?> code = (String O) -> {
				System.out.println("todo" + O);
				return Double.valueOf(1.5);
			};
			// Object x = code.apply("xxx"); // compile error
			// Object x = code.apply((Object) "xxx"); // compile error
			// Double x = code.apply("xxx"); // compile error
		}
		{
			Function<Object, Object> code = (O) -> {
				System.out.println("todo" + O);
				return Double.valueOf(1.5);
			};
			Object x = code.apply("xxx");
			x = code.apply((Object) "xxx");
			Double xx = (Double) code.apply("xxx");
		}
		{
			Function code = (O) -> {
				System.out.println("todo" + O);
				return 1.5;
			};
			Object x = code.apply("xxx");
			x = code.apply((Object) "xxx");
			Double xx = (Double) code.apply("xxx");
		}

	}
}

(Ooops, why the dangling brace?)

From the net, the question mark is a "wildcard" matching *everything* (except primitive types). So "?" and "Object" should be equivalent.

What part did I miss?

java

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