The mechanic was decent enough to be a good premise for a simple puzzle game.
However, the game has many problems that prevent it from being enjoyable.
The character jumps way too fast, it can take multiple tries to place the block where you want it because of how fast the character moves, and the poor choice of placing mechanic, and the character interacts weirdly with the environment.
Additionally, some of the puzzles rely less on good design, and more on exploiting the way the game works. For example, level 2. As far as I can tell, you need the block later, but at first it seems you need it at the start. You really don't, as long as you move over while you're falling, but that just feels kind of cheap.
The game has problems, but it also has positives. The concept is solid, the art is fairly pleasing, and playable game out in such short time itself is impressive.
With the wonky movement hammered out, a different way of placing the blocks, and some improvements on the level design, and this could be a reasonably satisfying game.
Also nice to see you used Java!