The JVM considers the return type as part of the method signature.

Though it is not possible in Java, the JVM can have multiple methods with the same name and parameter types so long as they have different return types. The JVM spec seems to call it "method descriptor" but I called it signature as I think that's more attention grabbing (and I'm showing how the JVM spec and Java spec differ).

No two methods in one class file may have the same name and descriptor

You can experimentally test this yourself with the Jasmin project. Jasmin is a tool that lets you write assembly code to assemble to JVM byte code. I also used this classfileanalyzer project to get some base Jasmin assembly.

For the lazy, here is the Java code I started with,

public class Example {
  public static void main(String[] args) {
    System.out.println(foo());
    System.out.println(fooInt());
  }
  static String foo() {
    return "1";
  }
  static int fooInt() {
    return 1;
  }
}

Then I got this Jasmin code and replaced each fooInt with foo

; Example.j

; Generated by ClassFileAnalyzer (Can)
; Analyzer and Disassembler for Java class files
; (Jasmin syntax 2, http://jasmin.sourceforge.net)
;
; ClassFileAnalyzer, version 0.7.0


.bytecode 63.0
.source Example.java
.class public Example
.super java/lang/Object

.method public <init>()V
  .limit stack 1
  .limit locals 1
  .line 1
  0: aload_0
  1: invokespecial java/lang/Object/<init>()V
  4: return
.end method

.method public static main([Ljava/lang/String;)V
  .limit stack 2
  .limit locals 1
  .line 3
  0: getstatic java/lang/System/out Ljava/io/PrintStream;
  3: invokestatic Example/foo()Ljava/lang/String;
  6: invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V
  .line 4
  9: getstatic java/lang/System/out Ljava/io/PrintStream;
  12: invokestatic Example/foo()I
  15: invokevirtual java/io/PrintStream/println(I)V
  .line 5
  18: return
.end method

.method static foo()Ljava/lang/String;
  .limit stack 1
  .limit locals 0
  .line 7
  0: ldc "1"
  2: areturn
.end method

.method static foo()I
  .limit stack 1
  .limit locals 0
  .line 10
  0: iconst_1
  1: ireturn
.end method

After assembling Example.class with Jasmin and running javap -c Example I get this output,

Compiled from "Example.java"
public class Example {
  public Example();
    Code:
       0: aload_0
       1: invokespecial #28                 // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #33                 // Field java/lang/System.out:Ljava/io/PrintStream;
       3: invokestatic  #32                 // Method foo:()Ljava/lang/String;
       6: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       9: getstatic     #33                 // Field java/lang/System.out:Ljava/io/PrintStream;
      12: invokestatic  #11                 // Method foo:()I
      15: invokevirtual #24                 // Method java/io/PrintStream.println:(I)V
      18: return

  static java.lang.String foo();
    Code:
       0: ldc           #27                 // String 1
       2: areturn

  static int foo();
    Code:
       0: iconst_1
       1: ireturn
}

And of course, running Example.class outputs

1
1

I hope you've enjoyed this interesting quirk of the JVM. It is a good reminder that while the JVM is primarily for Java, there are other languages that use it and some might even use this in a meaningful way somehow.