ClassCastException occurs when migrating from Java7 to Java8 ~ Generics and overload ~

Self-introduction

• In-house Web system development with Terasoluna Server for Java (Java7, Java8): 4 years --NTT open source http://www.terasoluna.jp/product/framework/server_framework_5.html

• In-house Web system development with Spring Boot: 6 months

• Change job in April. Studying Scala

LT at Nagoya Java User Group April 2018!

Execution environment

• Java 1.8.0_162
• Java 1.7.0_80
• Eclipse 4.7.2(Pleiades)

table of contents

1. Problems that have occurred
2. Investigate the cause
3. Survey results

1. Problems that have occurred

What happens when you run it?

Main.java

public class Main {

	public static void main(String[] args) {
		String.valueOf(hoge());
	}

	static <E> E hoge() {
		return (E) "hoge";
	}

}

Java7 ends normally, Java8 causes ClassCastException

• Java 1.7.0_80: Normal termination
• Java 1.8.0_162: Exception "java.lang.String cannot be cast to char array" occurs

Console

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to [C
	at jp.co.sample.Main.main(Main.java:13)

[C is a char array https://docs.oracle.com/javase/specs/jls/se7/html/jls-10.html#jls-10.8

What is this source in the first place?

• Existing code for a web system created around 2000 (source modified for announcement)
• In response to aging updates in Java8, there was a problem that "execution results are different due to differences in minor versions of Java8". --Since I forgot the minor version of Java 8 that did not cause an error, I compared Java 7 and Java 8 in this announcement.

Problems with this source

• Wrong usage of generics --No type parameters required
• String.valueOf has no meaning

Don't worry about this announcement.

2. Investigate the cause

Check with Eclipse

The compiler is ECJ (Eclipse Java Compiler), which is different from the JDK, but the result is the same.

In Java 7, the hoge type parameter is ʻObject`

In Java8 the hoge type parameter is char []

char [] comes from String.valueOf.

String.valueOf is overloaded

Can accept char array type or Object type arguments

• valueOf(char[] data)
• valueOf(Object obj)

Situation arrangement

• Let the result of the generics method be the value A --The return value is a temporary parameter --Since there is no argument, the actual parameter is unknown
• The value A is passed to the method that can receive the argument of char array type or Object type.

Checking generic methods

• If the type parameter is decided (String), java8 will also end normally.

MainGenericsMethod.java

public class MainGenericsMethod {

	public static void main(String[] args) {
		String.valueOf(fuga("fuga"));
	}

	/**The type parameter is determined from the argument*/
	static <E> E fuga(E arg) {
		return (E) arg;
	}
}

Check for overload

The result of executing in Java 8 by changing the combination of overloaded methods.

• Object and char []: NG
• Object and ʻint [] `: NG
• Object and char: OK
• Object and ʻint`: OK

The problem is an overloaded method that accepts an Object or array type.

MainOverload.java

public class MainOverload {

	public static void main(String[] args) {
		overload(hoge());
	}

	static <E> E hoge() {
		return (E) "hoge";
	}

	static void overload(Object arg) {}
	static void overload(char[] arg) {} //Change the argument type
}

3. Survey results

Summary

• The behavior when passing a value whose type parameter is unknown to an overloaded method that receives an array or Object depends on the Java version.
• In Java 1.7.0_80, the method that receives Object type is executed.
• In Java 1.8.0_162, the method that receives the array type is executed.

[Supplement] Ambiguous overload

When I call an overloaded method that receives an ʻint [] or char [] `, I get a compile error that says" Method overload (Object) is of type MainOverload is ambiguous "

        //Compile error
	static void overload(int[] arg) {}
	static void overload(char[] arg) {}

Articles that may be related to this issue

• String.valueOf gives ClassCastException

So, whereas before Java 8 the method argument site did not receive any inference, defaulting to Object, in Java 8 the most specific applicable type is inferred, in this case String.

• Why does the Java 8 generic type inference pick this overload?

• Why is this method overloading ambiguous?

[Supplement] I tried disassembling with Java 8 javap -verbose

Classfile /C:/Users/yuji3/Desktop/java-test/java8/Main.class
  Last modified 2018/04/14; size 476 bytes
  MD5 checksum 409af5d1b1986da6e0c4cd431dc50b59
  Compiled from "Main.java"
public class Main
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #7.#20         // java/lang/Object."<init>":()V
   #2 = Methodref          #6.#21         // Main.hoge:()Ljava/lang/Object;
   #3 = Class              #22            // "[C"
   #4 = Methodref          #23.#24        // java/lang/String.valueOf:([C)Ljava/lang/String;
   #5 = String             #14            // hoge
   #6 = Class              #25            // Main
   #7 = Class              #26            // java/lang/Object
   #8 = Utf8               <init>
   #9 = Utf8               ()V
  #10 = Utf8               Code
  #11 = Utf8               LineNumberTable
  #12 = Utf8               main
  #13 = Utf8               ([Ljava/lang/String;)V
  #14 = Utf8               hoge
  #15 = Utf8               ()Ljava/lang/Object;
  #16 = Utf8               Signature
  #17 = Utf8               <E:Ljava/lang/Object;>()TE;
  #18 = Utf8               SourceFile
  #19 = Utf8               Main.java
  #20 = NameAndType        #8:#9          // "<init>":()V
  #21 = NameAndType        #14:#15        // hoge:()Ljava/lang/Object;
  #22 = Utf8               [C
  #23 = Class              #27            // java/lang/String
  #24 = NameAndType        #28:#29        // valueOf:([C)Ljava/lang/String;
  #25 = Utf8               Main
  #26 = Utf8               java/lang/Object
  #27 = Utf8               java/lang/String
  #28 = Utf8               valueOf
  #29 = Utf8               ([C)Ljava/lang/String;
{
  public Main();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 2: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=1, args_size=1
         0: invokestatic  #2                  // Method hoge:()Ljava/lang/Object;
         3: checkcast     #3                  // class "[C"
         6: invokestatic  #4                  // Method java/lang/String.valueOf:([C)Ljava/lang/String;
         9: pop
        10: return
      LineNumberTable:
        line 4: 0
        line 5: 10

  static <E extends java.lang.Object> E hoge();
    descriptor: ()Ljava/lang/Object;
    flags: ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: ldc           #5                  // String hoge
         2: areturn
      LineNumberTable:
        line 8: 0
    Signature: #17                          // <E:Ljava/lang/Object;>()TE;
}
SourceFile: "Main.java"

[Supplement] I tried disassembling with Java 7 javap -verbose

Classfile /C:/Users/yuji3/Desktop/java-test/java7/Main.class
  Last modified 2018/04/14; size 481 bytes
  MD5 checksum 72c2eee8d5b013579f81dc56f53458d0
  Compiled from "Main.java"
public class Main
  SourceFile: "Main.java"
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER

Constant pool:
   #1 = Methodref          #6.#19         //  java/lang/Object."<init>":()V
   #2 = Methodref          #5.#20         //  Main.hoge:()Ljava/lang/Object;
   #3 = Methodref          #21.#22        //  java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
   #4 = String             #13            //  hoge
   #5 = Class              #23            //  Main
   #6 = Class              #24            //  java/lang/Object
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               main
  #12 = Utf8               ([Ljava/lang/String;)V
  #13 = Utf8               hoge
  #14 = Utf8               ()Ljava/lang/Object;
  #15 = Utf8               Signature
  #16 = Utf8               <E:Ljava/lang/Object;>()TE;
  #17 = Utf8               SourceFile
  #18 = Utf8               Main.java
  #19 = NameAndType        #7:#8          //  "<init>":()V
  #20 = NameAndType        #13:#14        //  hoge:()Ljava/lang/Object;
  #21 = Class              #25            //  java/lang/String
  #22 = NameAndType        #26:#27        //  valueOf:(Ljava/lang/Object;)Ljava/lang/String;
  #23 = Utf8               Main
  #24 = Utf8               java/lang/Object
  #25 = Utf8               java/lang/String
  #26 = Utf8               valueOf
  #27 = Utf8               (Ljava/lang/Object;)Ljava/lang/String;
{
  public Main();
    flags: ACC_PUBLIC

    Code:
      stack=1, locals=1, args_size=1
         0: aload_0       
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return        
      LineNumberTable:
        line 2: 0

  public static void main(java.lang.String[]);
    flags: ACC_PUBLIC, ACC_STATIC

    Code:
      stack=1, locals=1, args_size=1
         0: invokestatic  #2                  // Method hoge:()Ljava/lang/Object;
         3: invokestatic  #3                  // Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
         6: pop           
         7: return        
      LineNumberTable:
        line 4: 0
        line 5: 7

  static <E extends java/lang/Object> E hoge();
    flags: ACC_STATIC

    Code:
      stack=1, locals=0, args_size=0
         0: ldc           #4                  // String hoge
         2: areturn       
      LineNumberTable:
        line 8: 0
    Signature: #16                          // <E:Ljava/lang/Object;>()TE;
}