String placehodler = 'Hello {0} , {1} is cool!';

List<String> fillers = new String[]{'Jason','Apex'};

String formatted = String.format(placehodler,fillers);

System.assertEquals('Hello Jason , Apex is cool!',formatted);

Anonymous execution was successful.

38.0 APEX_CODE,DEBUG;APEX_PROFILING,INFO;CALLOUT,INFO;DB,INFO;SYSTEM,DEBUG;VALIDATION,INFO;VISUALFORCE,INFO;WORKFLOW,INFO

Execute Anonymous: String placehodler = 'Hello {0} , {1} is cool!';

Execute Anonymous: List<String> fillers = new String[]{'Jason','Apex'};

Execute Anonymous: String formatted = String.format(placehodler,fillers);

Execute Anonymous: System.assertEquals('Hello Jason , Apex is cool!',formatted);

13:50:33.21 (21991712)|USER_INFO|[EXTERNAL]|0056F00000652OT|weizhen.zhao@pactera.com|中国标准时间|GMT+08:00

13:50:33.21 (22007848)|EXECUTION_STARTED

13:50:33.21 (22013825)|CODE_UNIT_STARTED|[EXTERNAL]|execute_anonymous_apex

13:50:33.22 (22913817)|CUMULATIVE_LIMIT_USAGE

13:50:33.22 (22913817)|LIMIT_USAGE_FOR_NS|(default)|

  Number of SOQL queries: 0 out of 100

  Number of query rows: 0 out of 50000

  Number of SOSL queries: 0 out of 20

  Number of DML statements: 0 out of 150

  Number of DML rows: 0 out of 10000

  Maximum CPU time: 0 out of 10000

  Maximum heap size: 0 out of 6000000

  Number of callouts: 0 out of 100

  Number of Email Invocations: 0 out of 10

  Number of future calls: 0 out of 50

  Number of queueable jobs added to the queue: 0 out of 50

  Number of Mobile Apex push calls: 0 out of 10

13:50:33.22 (22913817)|CUMULATIVE_LIMIT_USAGE_END

13:50:33.21 (22953824)|CODE_UNIT_FINISHED|execute_anonymous_apex

13:50:33.21 (25331707)|EXECUTION_FINISHED