In C++ you can inherit from types that doesn't share common ancestor and avoid the "dreaded diamond" problem altogether. In fact this is what most of C++ code do: design around the problem. When there is no other option, then one can choose between virtual inheritance or non-virtual one. Each has it's own advantages and disadvantages, but none is a truly "successfully addressed" solution.

In Java, every class inherits Object implicitly. Now this means that every multiple inheritance will create the "dreaded diamond" right away! Which means that even simple hierarchy would obligatory turn into a nightmare. This is where interface concept comes to the rescue. In C++ interface is just another class. In Java, interface is unique by not inheriting from Object. Java solves the "dreaded diamond" problem in same way that 99% of C++ code does it: by not letting diamonds to happen in the first place.

Bottom line: introducing diamond inheritance to Java would help in very few cases at a huge cost of allowing very difficult to understand code.

You must keep in mind that the biggest advantage that Java holds over C++ is that it doesn't let programmers to play with dangerous features that are mostly unnecessary (in jargon "shooting oneself in the foot"). Don't ask "why we don't have X", ask yourself "which types of problems cannot be solved without X". Most of the time you'd get a simple answer: "none". Or "none that matter to overwhelming majority of Java users".

In C++ you can inherit from types that doesn't share common ancestor and avoid the "dreaded diamond" problem altogether. In fact this is what most of C++ code do: design around the problem. When there is no other option, then one can choose between virtual inheritance or non-virtual one. Each has its own advantages and disadvantages, but none is a truly "successfully addressed" solution.

In Java, every class inherits Object implicitly. Now this means that every multiple inheritance will create the "dreaded diamond" right away! Which means that even simple hierarchy would obligatory turn into a nightmare. This is where interface concept comes to the rescue. In C++ interface is just another class. In Java, interface is unique by not inheriting from Object. Java solves the "dreaded diamond" problem in same way that 99% of C++ code does it: by not letting diamonds to happen in the first place.

Bottom line: introducing diamond inheritance to Java would help in very few cases at a huge cost of allowing very difficult to understand code.

You must keep in mind that the biggest advantage that Java holds over C++ is that it doesn't let programmers to play with dangerous features that are mostly unnecessary (in jargon "shooting oneself in the foot"). Don't ask "why we don't have X", ask yourself "which types of problems cannot be solved without X". Most of the time you'd get a simple answer: "none". Or "none that matter to overwhelming majority of Java users".