Merge pull request cheat#43 from Jonny-exe/master

Initial cheatsheet for java

java

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+---
+tags: [ java, jar ]
+---
+# To execute a java program
+# To run a java file
+java -jar <filename.jar>
+
+# To pass arguments to your java program
+java -jar <filename.jar> <arg1> <arg2> ...
+java -jar example.jar "Hello world" 1234
+
+# To get the version of the installed java enviroment
+java -showversion
+
+Example output:
+openjdk version "11.0.8" 2020-07-14
+OpenJDK Runtime Environment (build 11.0.8+10-post-Ubuntu-0ubuntu120.04)
+OpenJDK 64-Bit Server VM (build 11.0.8+10-post-Ubuntu-0ubuntu120.04, mixed mode, sharing)
+
+# To set the intial memory size to be used by the program
+# Use -Xms<size> to set inital memory allocation
+# Use -Xmx<size> to set maximun allowed memory allocation
+# Use -Xss<size> to set maximun allowed thread stack size
+# Initially asigns 256mb and allows up to 2gb, thread stack size of 1mb
+java -Xms256m -Xmx2g -Xss1m -jar <filename.jar>
+
+# To use the classpath variable
+# It overrides the CLASSPATH of Environment variable but only for that session. 
+# If you restart the application you need to again set the classpath variable.
+# You can use either: -classpath, --classpath or -cp
+java -classpath <classpath> -jar <filename.jar>
+java -classpath "my/example/path/one:my/example/path/two" -jar myfile.zip
+
+# To execute a java class
+# Say you have a java class of name HelloWorld in a file called HelloWorld.java
+# Say you want to execute your class HelloWorld and pass it the argument "hello"
+# Say the file is the local directory we use the options "-cp ."
+java -cp <path> <class> <arg>
+java -cp . HelloWorld "hello"