It is not possible to install openJDK 10 in LEAP15.1 as additional version. 

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~/tmp> sudo zypper in java-10-openjdk
[sudo] password for root:
Loading repository data...
Reading installed packages...
Resolving package dependencies...

Problem: installed java-11-openjdk-11.0.3.0-lp151.3.3.1.x86_64 obsoletes java-10-openjdk < 11.0.3.0-lp151.3.3.1 provided by java-10-openjdk-10.0.2.0-lp151.2.3.2.x86_64
 Solution 1: Following actions will be done:
  downgrade of java-11-openjdk-11.0.3.0-lp151.3.3.1.x86_64 to java-11-openjdk-11.0.3.0-lp151.2.1.x86_64
  downgrade of java-11-openjdk-accessibility-11.0.3.0-lp151.3.3.1.x86_64 to java-11-openjdk-accessibility-11.0.3.0-lp151.2.1.x86_64
 Solution 2: do not install java-10-openjdk-10.0.2.0-lp151.2.3.2.x86_64

Choose from above solutions by number or cancel [1/2/c] (c):
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So it is not possible to change the Java-Environment via update-alternatives