package testpackage;

public interface MyInterface {
    default String def() {
        return "[" + toString() + ", " + "def]";
    }

    default String join(MyInterface other) {
        return def() + other.def();
    }
}

class C implements MyInterface{}  compiles to:

public class testpackage.MyClass implements testpackage.MyInterface {
  public testpackage.MyClass();
    Code:
       0: aload_0
       1: invokespecial #16                 // Method java/lang/Object."<init>":()V
       4: return

  public java.lang.String join(testpackage.MyInterface);
    Code:
       0: aload_0
       1: aload_1
       2: invokestatic  #46                 // Method testpackage/MyInterfacehelper.join:(Ltestpackage/MyInterface;Ltestpackage/MyInterface;)Ljava/lang/String;
       5: areturn

  public java.lang.String def();
    Code:
       0: aload_0
       1: invokestatic  #49                 // Method testpackage/MyInterfacehelper.def:(Ltestpackage/MyInterface;)Ljava/lang/String;
       4: areturn
}

Where testpackage.MyInterfacehelper is compiled to
public class testpackage.MyInterfacehelper {
  private testpackage.MyInterfacehelper();
    Code:
       0: aload_0
       1: invokespecial #9                  // Method java/lang/Object."<init>":()V
       4: return

  public static java.lang.String def(testpackage.MyInterface);
    Code:
       0: new           #13                 // class java/lang/StringBuilder
       3: dup
       4: invokespecial #14                 // Method java/lang/StringBuilder."<init>":()V
       7: ldc           #16                 // String [
       9: invokevirtual #20                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      12: aload_0
      13: invokevirtual #24                 // Method java/lang/Object.toString:()Ljava/lang/String;
      16: invokevirtual #20                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      19: ldc           #26                 // String ,
      21: invokevirtual #20                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: ldc           #28                 // String def]
      26: invokevirtual #20                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      29: invokevirtual #29                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      32: areturn

  public static java.lang.String join(testpackage.MyInterface, testpackage.MyInterface);
    Code:
       0: new           #13                 // class java/lang/StringBuilder
       3: dup
       4: invokespecial #14                 // Method java/lang/StringBuilder."<init>":()V
       7: aload_0
       8: invokeinterface #35,  1           // InterfaceMethod testpackage/MyInterface.def:()Ljava/lang/String;
      13: invokevirtual #20                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      16: aload_1
      17: invokeinterface #35,  1           // InterfaceMethod testpackage/MyInterface.def:()Ljava/lang/String;
      22: invokevirtual #20                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      25: invokevirtual #29                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      28: areturn
}

==============
interface StaticTest {
    static <T> T staticMethod(T t) {
        return t;
    }
}
compiles to
public class testpackage.StaticTesthelper {
  private testpackage.StaticTesthelper();
    Code:
       0: aload_0
       1: invokespecial #9                  // Method java/lang/Object."<init>":()V
       4: return

  public static <T> T staticMethod$static(T);
    Code:
       0: aload_0
       1: areturn
}

========

Brigde methods are generated properly in example:
public interface BridgeParent<T> {
    T get();
}
public interface StringBridges extends BridgeParent<String> {
    @Override
    default String get() {
        return "default method";
    }

    default String concrete() {
        return "concrete";
    }
}
public class testpackage.StringBridgeshelper
  SourceFile: "testpackage/StringBridges.java"
  minor version: 0
  major version: 50
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Utf8               testpackage/StringBridgeshelper
   #2 = Class              #1             //  testpackage/StringBridgeshelper
   #3 = Utf8               java/lang/Object
   #4 = Class              #3             //  java/lang/Object
   #5 = Utf8               testpackage/StringBridges.java
   #6 = Utf8               <init>
   #7 = Utf8               ()V
   #8 = NameAndType        #6:#7          //  "<init>":()V
   #9 = Methodref          #4.#8          //  java/lang/Object."<init>":()V
  #10 = Utf8               get
  #11 = Utf8               (Ltestpackage/StringBridges;)Ljava/lang/String;
  #12 = Utf8               default method
  #13 = String             #12            //  default method
  #14 = Utf8               concrete
  #15 = String             #14            //  concrete
  #16 = Utf8               (Ltestpackage/StringBridges;)Ljava/lang/Object;
  #17 = Utf8               testpackage/StringBridges
  #18 = Class              #17            //  testpackage/StringBridges
  #19 = Utf8               ()Ljava/lang/String;
  #20 = NameAndType        #10:#19        //  get:()Ljava/lang/String;
  #21 = InterfaceMethodref #18.#20        //  testpackage/StringBridges.get:()Ljava/lang/String;
  #22 = Utf8               Code
  #23 = Utf8               LineNumberTable
  #24 = Utf8               SourceFile
{
  private testpackage.StringBridgeshelper();
    descriptor: ()V
    flags: ACC_PRIVATE
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #9                  // Method java/lang/Object."<init>":()V
         4: return

  public static java.lang.String get(testpackage.StringBridges);
    descriptor: (Ltestpackage/StringBridges;)Ljava/lang/String;
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=1, args_size=1
         0: ldc           #13                 // String default method
         2: areturn
      LineNumberTable:
        line 9: 0

  public static java.lang.String concrete(testpackage.StringBridges);
    descriptor: (Ltestpackage/StringBridges;)Ljava/lang/String;
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=1, locals=1, args_size=1
         0: ldc           #15                 // String concrete
         2: areturn
      LineNumberTable:
        line 13: 0

  public static java.lang.Object get(testpackage.StringBridges);
    descriptor: (Ltestpackage/StringBridges;)Ljava/lang/Object;
    flags: ACC_PUBLIC, ACC_STATIC, ACC_BRIDGE, ACC_SYNTHETIC
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokeinterface #21,  1           // InterfaceMethod testpackage/StringBridges.get:()Ljava/lang/String;
         6: areturn
      LineNumberTable:
        line 6: 0
}