The /kill command chat output will display the entity type and UUID when hovered over. The /testfor output doesn't. Neither do any scoreboard commands.
Code analysis by @unknown can be found in this comment.
Environment
Operating system: OSX 10.9.5
Java Version: 1.6.0_65
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Comments 5
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The following is based on decompiled version of Minecraft 1.8 using MCP. All method and class names are the names used in the decompiled version.
The reason why this happens is that multiple commands call the public String getName() method (returns plain text) of the net.minecraft.entity.Entity class instead of the public IChatComponent getDisplayName() method.
Affected commands are (in 1.8):
/clear/effect/execute/gamemode/give/playsound/kick/spawnpoint/xp/achievement/ban-ip1/ban1/scoreboard1/tp/testforCommand notifications for admins
1 The player name as String is here used to execute the command but to display the entity name as well
Tested, looks like it's true. When the /testfor command is used, it does not show the UUID for all entities.